! LIBRARY 
JNIVERSITY  OF 
CALIFORNIA 


EARTH 

SCIENCES 

LIBRARY 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 


IN  MEMORY  OF 

PROFESSOR 
GEORGE  D.  LOUDERBACK 

1874-1957 


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Sold  by  Book  Department 
MINING  AND  SCIENTIFIC  PRESS 

4|?MMARKET    STRE" 

SAN   FRANCfSCO 

"eof  Technical  Books  on  «.„„„ 


STRUCTURAL  GEOLOGY 


BY 

C.  K.  LEITH 

UNIVERSITY    OF    WISCONSIN 


NEW  YORK 

HENRY  HOLT  AND  COMPANY 
1913 


COPYRIGHT,  1913 

BY 
HENRY  HOLT  AND  COMPANY 


PRESS  OF  T.   MORKY  &  SON, 
GREENFIELD,  MASS  ,  U     S    A. 


QEsoi 


EARTH 

INTRODUCTION 


The  central  feature  of  structural  geology  is  the  interpretation  of 
structures  produced  in  rocks  by  earth  movements.  The  outer 
limits  of  structural  geology  are  not  clearly  denned,  for  in  one  way 
or  another  the  subject  is  interrelated  with  nearly  all  phases  of 
geology.  Its  purpose  is  the  study  and  interpretation  of  rock 
structures,  not  for  themselves,  but  for  the  light  they  may  throw  on 
stratigraphic  problems,  on  economic  geology,  on  the  causes  under- 
lying the  general  configuration  of  the  earth,  and  on  earth's  history. 

The  structural  geologist  has  in  recent  years  found  it  necessary 
in  his  field  work  to  give  much  attention  to  the  genetic  relationships 
of  rock  structures  produced  by  deformation.  Some  of  these  rela- 
tionships have  not  yet  found  expression  in  the  available  literature 
on  the  subject.  The  student  reads  in  general  text-books  about 
individual  structures  but  seldom  of  their  relations,  with  the  result 
that  at  least  in  his  early  field  work  he  may  fail  to  utilize  methods 
which  are  helpful  or  essential  in  the  interpretation  of  the  geology  of 
a.  district.  Emphasis  upon  geological  structures  as  related  parts 
of  a  record  or  process  rather  than  as  isolated  facts  determines 
the  method  of  presentation  in  this  book.  Illustrations  are  chosen 
principally  from  the  United  States. 

Primary  structures  of  rocks,  such  as  bedding  and  igneous  struc- 
tures, are  to  be  considered  in  the  study  of  structural  geology,  but 
these  receive  more  or  less  adequate  treatment  in  stratigraphic  and 
petrographic  geology.  The  writer  will  therefore  treat  these  sub- 
jects only  incidentally,  putting  the  emphasis  on  secondary  struc- 
tures developed  in  rocks  by  earth  movements. 

The  writer  is  indebted  to  Professor  Eliot  Blackwelder  of  the 
University  of  Wisconsin  for  several  of  the  illustrative  examples  of 
the  expression  of  structures  on  the  erosion  surface,  and  to  Pro- 
fessor W.  J.  Mead  for  important  suggestions  relating  to  experi- 
mental deformation.  Greatest  of  all  is  the  writer's  obligation  to 
President  C.  R.  Van  Hise,  who  as  teacher  and  associate  in  geologi- 

iii 

119 


iv  INTRODUCTION 

cal  field  work  originated  and  developed  many  of  the  ideas  expressed 
in  this  book.  For  some  years  the  structural  discussion  in  Van 
Rise's  "  Principles  of  North  American  Pre-Cambrian  Geology"  1 
has  been  widely  used  by  American  teachers  of  structural  geology. 
The  writer  had  the  privilege  of  association  with  Dr.  Van  Hise  in  the 
development  of  that  work,  and  the  present  volume  is  partly  a 
development  and  revision  of  the  ideas  of  that  paper. 

1  Van  Hise,  C.  R.,  Principles  of  North  American  Pre-Cambrian  Geology:  16th 
Ann.  Kept.  U.  S.  Geol.  Survey,  pt.  1,  1896,  pp.  571-874. 


CONTENTS 


PAGE 

INTRODUCTION        .                                           .  iii 

FRACTURE  AND  FLOW             1 

KINDS  OF  FRACTURE   AND  FLOW  STRUCTURES           ....  1 

DISTRIBUTION  OF  FRACTURE  AND  FLOW  STRUCTURES         ...  1 

CONDITIONS  FAVORING  FRACTURE  OR  FLOW 4 

DEPTH  NECESSARY  FOR  ROCK  FLOW           ......  9 

VOLUME  CHANGES  IN  FRACTURE  AND  FLOW  .         .         .         .11 

SURFACE  EXPRESSION  OF  THE  ZONES  OF  FRACTURE  AND  FLOW            .  12 

FRACTURES 14 

ATTITUDES  OF  FRACTURES  WITH  REFERENCE  TO  STRESSES          .         .  14 

TENSION  FRACTURES 14 

COMPRESSION  FRACTURES 16 

JOINTS      ............  21 

Joints  which  can  be  classified  as  due  to  tension         ...  22 

Joints  which  can  be  classified  as  due  to  compression         .         .  23 

Joints  developed  under  unknown  stress-strain  conditions           .  28 

Widening  of  joints  by  the  linear  force  of  growing  crystals          .  29 

Surface  expression  of  joints       .......  30 

Suggestions  for  laboratory  work  on  joints          .         .         .         .31 

FAULTS 31 

Nomenclature 32 

Apparent  and  real  fault  displacements 36 

Normal  faults    . 39 

Normal  faults  associated  with  igneous  rocks          ...  42 
Normal  faults  in  unfolded  sediments             ....  43 
Association  of  normal  faults  with  folds         ....  43 
Vertical  and  steeply-dipping  normal  faults  and  joints  in  in- 
tersecting systems          .......  44 

Reverse  or  thrust  faults 46 

Distributive  thrust  faults      .......  48 

Faults  with  horizontal  displacements         .....  50 

Hinge  or  pivotal  faults 50 

Curved  and  folded  faults  .         .         .         .         .         .         .51 

Faults  passing  into  folds  or  into  schistose  zones         .         .         .51 

Correlation  of  faults 53 

Relative  number  of  normal  and  reverse  faults            ...  54 
Relative  shortening  and  elongation  of  the  earth's  crust  by 

faulting      .....                           ...  55 


vi  CONTENTS 

FAULTS — Continued  PAGE 

Evidence  of  faulting  ........       56 

Surface  expression  of  faults       .......       57 

Suggestions  for  laboratory  study  of  faults          ....       60 

FRACTURE  CLEAVAGE  AND  FISSILITY          .         .         .         .         .         .61 

BRECCIAS  AND  AUTOCLASTICS    ........       64 

EARTHQUAKES  ..........       67 

Earthquakes  as  cause  and  effect  of  rock  fracture       ...       67 
Kinds  of  fracturing  accompanying  earthquakes         ...       69 

Earthquakes  and  oscillations  of  glaciers 70 

Earthquakes  and  vulcanism      .         .         .         .         .  .70 

Earthquakes  and  magnetic  disturbances  .         .         .         .70 

Earthquakes  and  rock  density  .         .         .         .         .         .70 

Earthquake  zones      .         .         .         .         .         .         .         .         .71 

Instruments  for  determining  and  measuring  earthquakes  .       71 

Earthquake  waves    .         .         .         .         .         .         .         .         .       72 

Condition   of   earth's  interior   as   inferred   from   earthquake 

waves         ..........       72 

Location  of  the  origin  of  earthquakes 73 

Prediction  of  earthquakes          .......       74 

ROCK   FLO  WAGE       .         . .       76 

FLOW  CLEAVAGE 76 

Manner  in  which  the  parallel  arrangement  of  minerals  is  brought 

about 79 

Recry  stabilization          ........       79 

Granulation  and  rotation  of  original  particles       ...       82 

Cleavage  in  its  relations  to  differential  pressures       ...       84 

Relations  of  cleavage  to  strain       ......       85 

Relations  of  cleavage  to  stress       ......       86 

GNEISSIC  STRUCTURE 87 

IDIOMORPHIC  OR  PORPHYRITIC  TEXTURES  DEVELOPED  BY  ROCK  FLOW- 
AGE  ....  .90 

ROCK  FLOW  AGE  WITHOUT  RETENTION  OF  CLEAVAGE          ...      92 

OBLITERATION  OF  TEXTURES  BY  ROCK  FLOWAGE        ....       93 

IDENTIFICATION  OF  SCHISTS  AND  GNEISSES        .....       97 

Field  relation  as  a  means  of  identifying  schists  and  gneisses       97 

Mineral  composition  as  a  means  of  identifying  schists  and 

gneisses .98 

Chemical  composition  as  a  means  of  identifying  igneous  or  sed- 
imentary origin  of  gneisses  and  schists       .    -     .         .         .100 
Conclusion  as  to  methods  of  identifying  gneisses  and  schists     102 

STRUCTURES  COMMON  TO  BOTH  FRACTURE  AND  FLOW  .     104 

FOLDS .104 

Elements  of  folds .104 

Folds  in  the  zone  of  fracture  and  zone  of  flow  contrasted  .     108 


CONTENTS  vii 

STRUCTURES  COMMON  TO  BOTH  FRACTURE  AND  FLOW— 

Continued  PAGE 

Control  of  structures  in  weak  beds  by  differential  movements 

between  competent  beds  on  limbs  of  folds          .         .         .  114 

(1)  Minor  folds   as  evidence   of  differential   movement   be- 

tween beds  .         .         .         .         .         .         .         .114 

(2)  Cleavage  as  evidence  of  differential  movement  in  folding  119 

(3)  Jointing,  fracture  cleavage,  and  fissility  as  evidences  of 
differential  movement  between  beds  in  folding         .         .121 

Localization  of  folds          .         .         .         .         .         .         .         .124 

Determination  of  depth  affected  by  folds           ....  124 

Field  observations  on  folds 127 

Strike  and  dip 127 

Emphasis  on  relations  of  major  and  minor  structures    .         .  128 

Field  observations  on  relations  of  cleavage  to  folds         .         .  128 
Determination  of  top  and  bottom  of  sedimentary  beds  in  a 

folded  area 132 

Suggestions  for  laboratory  study  of  folds           ....  134 

MOUNTAINS        ....                          136 

TYPES  OF  MOUNTAINS 136 

MOUNTAINS  AND  NORMAL  FAULTS 137 

MOUNTAINS  AND  THRUST  FAULTS 137 

MOUNTAINS  AND  FOLDS    .........  137 

MORE  COMPLEX  RELATIONS  OF  MOUNTAINS  TO  STRUCTURE         .         .  138 

LOCALIZATION  OF  MOUNTAINS 138 

SUGGESTIONS  FOR  LABORATORY  STUDY  OF  MOUNTAINS       .         .         .  140 

MAJOR  UNITS  OF  STRUCTURE  (GEANTICLINES,  GEOSYN- 
CLINES,  OCEAN  BASINS,  CONTINENTS,  PLATEAUS,  POSI- 
TIVE AND  NEGATIVE  ELEMENTS) 141 

SHAPES  OF  MAJOR  ELEMENTS  OF  STRUCTURE      .....  141 

ACTUAL  AND  APPARENT  UPLIFTS 143 

ULTIMATE  FORCES  OF  SECONDARY  DEFORMATION      .         .  144 

OUTLINE  OF  PRINCIPAL  THEORIES 144 

ISOSTASY .    .  145 

Support  of  hypothesis  by  recognition  of  weakness  of  rocks        .  145 

Dutton's  and  Gilbert's  observations  on  isostasy         .         .         .  146 

Hayford's  observations  on  isostasy    ......  146 

Earth  movements  in  relation  to  isostasy   .....  148 

Isostasy  in  relation  to  rigidity  of  rocks 148 

Depth  of  isostatic  compensation 149 

Criticism  of  theory  of  isostasy  .         .         .         .         .         .149 

CAUSES  OF  TENSION 152 

CONCLUSION  AS  TO  MAJOR  CAUSES  OF  DEFORMATION          .        .         .  152 

LOCAL  AND  MINOR  CAUSES  OF  DEFORMATION 153 

RELATION  BETWEEN  DEFORMATION  AND  VTTLCANISM  154 


viii  CONTENTS 


PAGE 

UNCONFORMITY 156 

IDENTIFICATION  OF  UNCONFORMITY  .         .  .         .         .  157 

INTERPRETATION  OF  UNCONFORMITY 159 

SUGGESTIONS  FOR  LABORATORY  STUDY  OF  UNCONFORMITY         .         .  161 

INDEX  163 


STRUCTURAL  GEOLOGY 


FRACTURE  AND  FLOW 

KINDS   OF   FRACTURE   AND   FLOW   STRUCTURES 

Rocks  are  deformed  by  fracture  and  by  flowage.  Rock  fractures 
are  known  geologically  as  joints,  faults,  brecciation,  autoclastic 
structures,  fracture  cleavage,  etc. 

Rock  flowage  may  be  defined  as  a  permanent  change  of  form  by 
pressure  without  conspicuous  fracture.  It  does  not  include  igneous 
fusion.  It  is  accomplished  by  interior  readjustments  of  rock 
substances  by  chemical,  mineralogical,  and  mechanical  changes, 
these  changes  being  favored  by  high  pressure  and  temperature, 
moisture,  and  by  the  presence  of  rock  substance  easily  susceptible 
to  these  changes.  The  results  of  rock  flowage  are  commonly  a 
parallel  arrangement  of  the  constituents  of  the  rock  mass,  produc- 
ing a  schistosity,  cleavage,  or  banded  structure.  Where  the  rock 
is  made  up  of  minerals  not  adapted  dimensionally  to  taking  on  a 
parallel  arrangement,  rock  flowage  may  leave  no  evidence  of  itself 
in  parallel  arrangement.  There  are  gradational  structures  between 
flow  and  fracture,  for  rock  may  be  deformed  mainly  by  minute 
fracture  or  slicing  and  still  be  a  coherent  mass.  It  has,  in  effect 
flowed. 

Folds  are  developed  by  both  flowage  and  fracture. 

DISTRIBUTION    OF    FRACTURE    AND    FLOW 
STRUCTURES 

The  prevailing  manner  of  deformation  at  the  earth's  surface  is 
by  fracture,  as  is  known  by  observation  and  experiment. 

The  prevailing  manner  of  deformation  deep  below  the  surface 
may  be  inferred  to  be  by  flowage.  Rock  flowage  has  been  actually 
observed  in  process,  as,  for  instance,  the  flowage  of  schists  in  the 
Simplon  and  other  deep  tunnels  and  the  creep  of  soft  shales  in 

1 


2  STRUCTURAL   GEOLOGY 

mines.  For  the  most  part,  however,  rock  flowage  takes  place  at 
depths  beyond  our  range  of  observation,  and  our  conclusions  as  to 
the  existence,  locus,  and  conditions  of  a  zone  of  rock  flowage  rest 
principally  on  inference.  We  observe  some  rocks  at  the  earth's 
surface  with  textures  which  have  been  developed  by  rock  flowage. 
We  see  that  flowage  is  not  now  taking  place  in  them.  We  know 
that  the  rocks  were  once  far  below  the  surface  and  now  appear  at 
the  surface  because  erosion  has  uncovered  them.  We  conclude 
that  they  flowed  when  beneath  the  surface,  under  physical  condi- 
tions other  than  those  under  which  they  now  rest,  and  that  there- 
fore rocks  are  today  flowing  beneath  the  surface.  We  reverse  the 
statement  of  the  Huttonian  principle  that  the  present  is  the  key 
to  the  past,  and  argue  that  the  past  is  the  key  to  the  present. 

Artificial  rock  flowage  may  be  accomplished  under  conditions 
which  seem  to  us  probably  analogous  to  those  existing  at  depth. 
(See  p.  4.) 

The  existence  of  a  zone  of  rock  flowage  beneath  the  surface  is 
inferred  also  from  the  behavior  of  earthquake  waves.  These  are 
initiated  by  the  shock  of  fracturing;  and  it  is  significant  that  their 
point  of  origin,  as  determined  by  many  independent  observers,  has 
never  been  found  to  be  far  below  the  surface.  This  fact  indicates 
that  fractures  go  only  to  a  comparatively  shallow  depth,  and  that 
below  this  rock  deformation  must  be  accomplished  in  some  other 
way. 

The  wrinkling  of  the  earth's  surface  into  mountain  ranges  in- 
volves a  slipping  of  the  crust  which  renders  plausible  the  existence 
of  a  zone  of  flow. 

If  the  earth's  surface  is  in  a  state  of  isostatic  adjustment,  the 
conclusion  seems  inevitable  that  this  adjustment  has  been  main- 
tained by  means  of  deep-seated  flowage  to  compensate  for  trans- 
ference of  surface  loads  by  erosion. 

It  is  concluded,  therefore,  partly  by  direct  observation  but 
largely  by  inference,  that  somewhere  beneath  the  earth's  crust  is  a 
zone  in  which  deformation  is  by  flowage.  It  may  seem  super- 
fluous to  use  so  many  words  to  argue  that  there  is  a  zone  of  rock 
flowage  yet  if  we  think  to  ask  ourselves  how  we  know  this,  we  are 
obliged  to  confess  that  inference  has  been  an  important  factor 
in  reaching  this  conclusion.  Actual  observation  does  not  go  be- 
low a  zone  of  combined  fracture  and  flow.  Even  the  Keewatin 


DISTRIBUTION   OF  STRUCTURES  3 

and  Laurentian  rocks,  the  oldest  of  the  pre-Cambrian  of  North 
America,  have  only  partly  undergone  rock  flowage,  and  even  in 
these  rocks  the  flowage  is  in  considerable  part  a  direct  result  of 
plutonic  intrusion  rather  than  depth  alone. 

The  existence  of  a  zone  of  fracture  and  a  zone  of  flow  was  in- 
ferred by  Heim1  from  his  studies  of  the  Alps.  Gilbert2  also  sep- 
arated the  two  types  of  deformation  on  basis  of  depth,  but  did  not 
use  the  term  zone.  Van  Hise3  first  proposed  a  classification  of  the 
lithosphere  on  a  basis  of  vertical  distribution  of  the  dominant 
kinds  of  deformation,  into  an  upper  zone  of  fracture,  a  middle 
zone  of  combined  fracture  and  flowage,  and  a  lower  zone  of  flowage. 
In  view  of  the  fact  that  flowage  in  certain  soft  rocks  may  begin 
almost  at  the  surface,  nearly  all  of  the  zone  of  the  lithosphere 
within  our  range  of  observation  is  that  of  combined  fracture  and 
flowage.  Also  rocks  which  have  been  deformed  by  flowage  below 
the  surface  in  the  past  and  are  now  exposed  by  erosion  lie  along- 
side of  rocks  now  being  fractured  at  the  surface  within  our  range  of 
observation.  The  depth  necessary  for  flowage  differs  for  different 
rocks,  and  is  dependent  upon  a  variety  of  conditions.  A  general 
statement  of  the  distribution  of  structures  is  that  at  the  surface 
most  rocks  fracture  and  some  flow ;  that  far  enough  below  the  sur- 
face all  rocks  may  flow.  Van  Hise  emphasized  the  variation  of 
depth  of  the  zones  of  fracture  and  flow  for  different  rocks  and  under 
different  conditions;  but  the  use  of  the  word  "zone"  has  caused  un- 
due stress  to  be  placed  on  uniformity  of  depth  by  students  who 
have  used  these  terms.  The  emphasis  should  rather  be  on  condi- 
tions. As  expressed  by  a  student  in  an  examination,  the  zone  of 
fracture  or  flowage  "like  Heaven,  is  a  condition,  not  a  place." 
If  "zone  "  were  understood  to  convey  the  notion  of  both  condition 
and  place,  it  would  more  clearly  express  the  fact.  A  hard  quart zite 
fractures,  while  a  shale  lying  either  above  or  below  may  flow.  A 
quartzite  may  fracture  at  one  place,  while  near  at  hand,  without 
increase  of  depth  but  under  different  conditions,  it  may  flow.  In 
order  that  the  terms  "zone  of  fracture"  and  "zone  of  flow"  may 
have  definite  significance,  they  should  be  related  to  specific  rocks, 

1  Heim,  Albert,  Untersuchungen  iiber  den   Mechanismus  der  Gebirgsbildung, 
Basel,  1878. 

2  Gilbert,  G.  K.,  Geology  of  the  Henry  Mountains:  2nd  ed.,  Washington,  1880. 

3  Van  Hise,  C.  R.,  Principles  of  North  American  Pre-Cambrian  Geology:  16th 
Ann.  Kept.  U.  S.  Geol.  Survey,  p.  589. 


4  STRUCTURAL   GEOLOGY 

for  instance,  "the  zone  of  fracture  for  quartzite,"  "the  zone  of  flow 
for  shale." 

As  rocks  approach  the  earth's  surface  by  erosion  of  overlying 
rocks  or  through  volcanic  agencies  they  become  fractured  and 
disintegrated.  As  they  are  buried  beneath  the  surface  they  may 
come  under  conditions  of  rock  flowage  which  weld  and  integrate 
them.  Structural  changes  may  thus  be  in  cycles.  As  the  depths 
of  fracture  and  flow  vary  widely  for  different  rocks  and  under 
different  conditions,  one  rock  may  be  in  the  destructive  phase  of  its 
structural  cycle  while  a  nearby  rock  may  be  in  a  constructive  phase. 
The  terms  "zone  of  fracture"  or  "zone  of  flow"  may  therefore  be 
considered  as  applying  to  a  given  rock  in  a  phase  of  its  structural 
cycle.  Depth  is  only  one  of  the  important  factors  determining  the 
phase  of  the  cycle. 

CONDITIONS  FAVORING  FRACTURE  OR  FLOW 

Most  rocks  fracture  at  the  surface ;  some  of  them  flow.  It  may 
be  supposed  that  far  enough  below  the  surface  all  of  them  may 
flow.  Practically,  our  zone  of  observation  is  that  of  combined 
fracture  and  flow.  These  kinds  of  deformation  may  occur  side  by 
side  in  different  rocks  or  in  the  same  rocks.  The  specific  combina- 
tion of  factors  which  determines  fracture  rather  than  flow  in  the 
given  location  can  seldom  be  more  than  approximately  ascertained. 

Rock  flowage  has  been  experimentally  accomplished  on  a  small 
scale.  Kick  l  in  1892  put  crystals  in  a  copper  box,  filled  the  space 
with  imbedding  material  such  as  paraffine  wax  and  fusible  metal, 
covered  the  box  with  brass  plates,  and  put  it  under  great  pressure. 
The  resistance  to  deformation  offered  by  the  copper  as  well  as  by 
the  imbedding  material  is  transmitted  through  the  bedding  ma- 
terial to  the  specimen,  which  thus  receives  a  very  considerable 
lateral  support.  In  this  manner  Kick  secured  permanent  deforma- 
tion in  salt,  talc,  gypsum,  fluorspar,  and  marble. 

Adams  subsequently  repeated  these  experiments  on  a  more 
elaborate  scale,  using  a  variety  of  limestones  and  marbles,  with 
similar  results.2  This  method  produces  rock  flowage.  The  essen- 

1  Kick,  Prof.  Friedrich,  Die  Prinzipien  der  mechanischen  Technologic  und  die 
Festigkeitslehre:  Zeit.  des  Ver.  Dcut.  Ingen.,  Vol.  36,  1892,  p.  919. 

2  Adams,  F.  D.,  An  experimental  investigation  into  the  action  of  differential 
pressure  on  certain  minerals  and  rocks,  employing  the  process  suggested  by  Pro- 
fessor Kick:  Jour,  of  Geol.,  Vol.  18,  No.  6,  1910,  pp.  489-525. 


CONDITIONS  OF  FRACTURE  OR  FLOW     5 

tial  condition  was  apparently  the  lateral  support.  The  method, 
however,  is  qualitative,  in  that  it  is  difficult  to  measure  the  pres- 
sure acting  upon  the  specimen  itself,  as  distinguished  from  that 
on  the  copper  box  and  on  the  paraffine. 

A  more  nearly  quantitative  method,  and  one  allowing  far  greater 
pressures,  has  been  used  by  Adams,1  who  fitted  cylinders  of  marble, 
granite,  and  diabase  into  steel  jackets  and  compressed  them  by  a 
piston  to  such  a  degree  that  the  sides  of  the  steel  casing  were  made 
to  bulge  (see  Fig.  1).  All  of  the  stresses  were  above  the  crushing 
strength  of  the  rocks,  but  they  differed  much  in  intensity.  When 
the  casing  had  been  cut  away  the  rock  was  found  to  have  nearly 
as  great  strength  as  it  had  before  deformation.  Similar  results 
have  been  observed  in  concrete  cylinders  incased  in  steel  jackets 
which  have  been  hardened  for  sixteen  hours  and  then  allowed  to 
stand  under  great  pressures.  The  result  was  deformation  by 
"flow."2 

Strength  tests  on  building  stone  cubes  afford  good  illustrations 
of  rock  fracture.  The  block  is  compressed  in  one  direction,  the 
sides  being  left  free.  The  maximum  pressure  required  for  fractur- 
ing the  strongest  rocks  is  from  25,000  to  30,000  pounds  per  square 
inch. 

In  the  most  general  terms,  experimental  results  seem  to  show 
that  when  a  rock  is  free  to  escape  in  some  direction,  it  will  break 
when  under  pressure  greater  than  its  crushing  strength.  When 
not  free  to  escape  except  by  exerting  a  pressure  greater  than  its 
crushing  strength,  it  flows  if  sufficient  pressure  is  brought  to  bear 
upon  it.  Expressed  more  technically,  the  pressure  acting  upon 
any  one  unit  of  the  rock  mass  may  be  resolved  into  three  mutually 
perpendicular  components,  called  the  three  principal  axes  of  stress. 
Where  one  or  two  of  these  axes  of  stress  are  less  than  the  crushing 
strength  of  the  rock  and  the  others  are  above  it,  the  rock  breaks, 
in  directions  determined  by  the  relative  intensities  of  the  three 
principal  stresses.  Where  all  of  the  stresses  are  greater  than  the 
crushing  strength  of  the  rock,  that  is,  when  the  rock  mass  is  con- 
fined on  all  sides  by  pressures  greater  than  its  crushing  strength, 

1  Adams,  Frank  D.,  and  Nicolson,  J.  T.,  An  experimental  investigation  into  the 
flow  of  marble:  Phil.  Trans.  Roy.  Soc.  of  London,  Vol.  195,  1901,  pp.  363-401.    See 
also,  Adams,  Frank  D.,  and  Coker,  Ernest  G.,  The  flow  of  marble:  Amer.  Jour. 
Sci.,  Vol.  29,  1910,  pp.  465-487. 

2  Engineering  News,  Vol.  54,  Nov.  2,  1905,  p.  459. 


STRUCTURAL  GEOLOGY 


FIG.  1.  Flowage  of  marble.    After  Adams,    a.  Columns  of  marble  before  and  after 
deformation,    b.  Deformed  column  of  marble  as  it  appears  in  the  steel  jacket. 

one  or  more  of  the  stresses  greatly  preponderating  over  the  others, 
the  rock  yields  by  rock  flowage. 

In  rock  flowage  the  stress-difference  (i.  e.,  difference  in  intensity 
of  greatest  and  least  of  the  principal  stresses)  necessary  to  deform 


CONDITIONS  OF  FRACTURE  OR  FLOW     7 

the  rock  may  be  much  greater  than  the  crushing  strength  of  the 
rock.  Experimental  evidence  points  in  this  direction,  although 
there  are  insufficient  data  to  warrant  satisfactory  quantitative 
statements.  Hallock1  has  shown  that  a  substance  like  a  dime  or 
a  brass  tack,  when  imbedded  in  steel  and  then  subjected  to  enor- 
mous pressure,  acquires  a  rigidity  which  allows  deformation  only 
when  the  stress  difference  has  become  very  large.  The  silver 
coin  acquires  so  great  a  rigidity  that  it  will  impress  itself  in  the 
steel  before  flowing.  Adams2  and  Pfaff 3  also  found  in  their  experi- 
ments that  when  rocks  were  under  pressure  enormously  greater 
than  their  ordinary  crushing  strength,  they  would  not  flow  through 
a  small  hole  bored  in  the  side  of  the  steel  jacket  nor  would  small 
holes  in  the  rock  become  closed;  and  it  was  concluded  that  a  high 
degree  of  artificial  rigidity  had  been  induced  in  the  rock,  which 
could  be  overcome  only  by  excessive  stress  difference.  High 
rigidity  would  seem  to  be  a  probable  condition  deep  in  the  earth, 
and  hence  enormous  stress  difference  might  be  required  to  effect 
deformation. 

While  under  certain  conditions  of  compression  the  rock  may 
flow,  it  may  fracture  under  tension  stresses  of  equal  or  greater 
magnitude.  The  breaking  strength  of  rocks  under  tension  is  less 
than  its  resistance  to  fracture  by  compression  or  to  flowage  by 
compression. 

A  substance  may  be  deformed  by  compressive  stress  at  the 
same  time  that  it  is  being  pulled  in  another  direction  by  a  tensional 
stress  (see.  pp.  16  and  25).  It  is  entirely  conceivable,  if  the  rock  is 
soft,  that  under  these  conditions  the  response  to  compression  may 
be  rock  flowage  and  the  response  to  tension  may  be  rock  fracture, 
for  it  is  known  that  under  tension  a  rock  breaks  under  much  less 
stress  than  under  compression,  and  under  the  higher  compressional 
stresses  there  may  be  rock  flowage. 

The  conditions  of  rock  flowage  in  the  earth  may  be  quite  different 
in  some  cases  from  those  experimentally  determined,  due  to  factors 
of  time,  moisture,  and  character  of  the  rock.  Given  long  enough 
time,  even  the  strongest  substances  may  become  deformed  without 

1  Hallock,  William,  The  flow  of  solids,  or  liquefaction  by  pressure:  Am.  Jour. 
Sci.,  Vol.  34,  1887,  p.  280. 

2  Adams,  Frank  D.,  An  experimental  contribution  to  the  question  of  the  depth 
of  the  zone  of  flow  in  the  earth's  crust:  Jour.  Geol.,  Vol.  20,  1912,  pp.  97-118. 

3  Pfaff,  F.,  Der  Mechanismus  der  Gebirgsbildung,  pp.  16-19. 


8  STRUCTURAL   GEOLOGY 

fracture  under  stresses  less  than  their  crushing  strength.  For 
instance,  marble  gravestones  sag  when  suspended  at  both  ends  for 
many  years.  Structural  materials  are  known  to  do  the  same.  In 
both  cases  the  load  is  less  than  that  necessary  for  crushing. 

Deformation  by  flowage  may  be  facilitated  by  high  tempera- 
ture and  moisture  content.  Such  factors  favor  rapid  chemical 
changes  and  recrystallization,  thereby  enabling  flow  to  take  place 
more  easily.  It  is  a  matter  of  observation  that  rocks  have  under- 
gone rock  flowage  by  means  of  recrystallization  of  the  mineral 
particles,  and  that  such  recrystallization  has  seemed  to  be  at  a 
maximum  in  rocks  which  were  once  at  a  high  temperature,  as 
near  intrusive  igneous  contacts,  or  had  a  high  content  of  moisture, 
or  both.  High  temperature  and  moisture  have  been  found  experi- 
mentally to  aid  recrystallization. 

Another  factor  which  helps  to  determine  fracture  or  flow  under 
given  conditions  is  the  character  of  the  rock  itself — its  weakness, 
and  its  susceptibility  of  recrystallization,  the  latter  in  turn  depend- 
ing on  mineral  content,  texture,  degree  of  hydration,  and  other 
conditions.  Thus  it  is  that  under  the  same  pressures  one  rock  may 
fracture  and  the  other  flow.  In  general,  muds,  shales,  slates,  and 
limestones  flow  much  more  readily  than  the  harder  types  such  as 
quartzite  and  igneous  rocks. 

The  scope  of  this  paper  does  not  call  for  any  attempt  to  explain 
the  physical  and  chemical  basis  of  recrystallization,  beyond  calling 
attention,  as  has  been  done,  to  the  general  factors  which  seem  to  l:e 
effective  according  to  field  and  experimental  observation.  Much 
remains  to  be  done  to  get  these  factors  on  a  quantitative  basis. 
It  is  entirely  likely  that  as  progress  is  made  in  this  regard  there  will 
be  a  considerable  change  in  the  emphasis  on  the  several  factors 
cited.  For  instance,  the  presence  of  moisture  seems  to  favor 
recrystallization,  judging  from  field  conditions.  Of  two  rocks  of 
different  moisture  content,  the  one  containing  the  more  water 
seems  to  recrystallize  more  readily,  yet  in  experiments  in  the 
artificial  recrystallization  of  minerals  in  the  Carnegie  Institution  of 
Washington  it  has  been  found  that  recrystallization  occurs  with 
unexpected  readiness  under  conditions  of  dry  heat.  Artificial 
rock  powders  when  heated  dry  have  been  found  to  recrystallize, 
giving  particles  large  enough  for  microscopic  study. 


DEPTH  OF  ROCK  FLOWAGE 


DEPTH  NECESSARY  FOR  ROCK  FLOW 

A  shale  may  be  deformed  by  flowage  near  the  surface,  while  a 
brittle  quartzite  may  require  great  depth.  A  rock  at  a  given 
depth  may  fracture  in  one  locality,  while  in  another  locality, 
because  of  vulcanism,  or  high  pressure  and  temperature  developed 
by  mechanical  thrust,  or  because  of  its  relations  to  adjacent 
strata,  may  be  deformed  by  flowage.  Therefore  no  one  figure 
may  be  taken  as  the  depth  of  the  zone  of  rock  fracture.  It  is 
apparent  that  the  depth  beneath  the  surface  necessary  to  produce 
rock  flowage  is  only  one  of  a  number  of  variable  factors  determin- 
ing the  manner  of  deformation  of  a  rock.  Among  these  are  the 
following:  whether  stresses  are  tensional  or  compressional,  varia- 
tion of  minor  compressive  stresses  and  thus  of  induced  rigidity, 
variation  in  strength  of  the  materials,  variation  in  chemical  and 
mineralogical  composition,  variation  in  moisture-content  and 
temperature,  duration  of  time,  and  possibly  other  unknown  varia- 
bles. Notwithstanding  our  lack  of  quantitative  measurements  of 
some  of  these  factors,  it  is  still  possible  to  arrive  at  some  approxima- 
tion for  the  minimum  depth  at  which  all  rocks  will  flow  even  when 
not  favored  by  factors  other  than  depth. 

An  early  attempt  to  use  quantitative  methods  in  determining 
this  depth  was  made  by  Van  Hise  and  Hoskins.1  Their  calculation 
of  the  depth  of  covering  which  would  give  a  pressure  sufficient 
to  close  a  cavity  gave  a  range  of  from  three  to  seven  miles.  In 
making  this  calculation  they  made  assumptions  favorable  to  the 
greatest  depth — for  instance,  that  the  rock  was  of  the  strongest 
known  kind,  that  conditions  of  temperature  and  moisture  were 
the  least  favorable  to  recrystallization,  that  lateral  stress  was 
absent,  that  the  pressure  is  lessened  by  the  buoying  effect  of  under- 
ground water.  One  of  their  assumptions,  however,  tends  to  make 
the  calculated  depth  too  small,  namely,  that  the  stress  difference 
necessary  to  close  a  cavity  is  just  equal  to  the  crushing  strength 
of  the  rock.  Experiments  of  Adams,  Pfaff,  and  Hallock,  cited 
above,  have  shown  that  the  rock  acquires  a  high  degree  of  rigidity 
when  compressed  on  all  sides,  and  that  enormously  greater  stress 
difference  is  necessary  to  cause  deformation  of  any  kind.  How 
much  greater  the  pressure  would  need  to  be  is  yet  uncertain. 

1  Op.  cit.,  pp.  589-593. 


10  STRUCTURAL  GEOLOGY 

Adams1  has  shown  experimentally  that  a  cavity  will  not  close 

11  miles  below  the  surface  at  a  temperature  of  550°  C.  even  if  a 
pressure  is  used  that  is  50%  greater  than  that  obtaining  at  this 
depth.     For  granite,  in  fact,  he  finds  that  cavities  remain  open 
at  ordinary  temperatures  even  with  pressures  corresponding  to 
depth  of  30  miles.    These  experiments  may  lead  to  overestimate  of 
depth  for  flowage  in  general,  for  the  reason  that  the  cavities  used 
were  very  minute,  the  factor  of  moisture  was  not  included,  and  the 
time  element  was  only  partially  accounted  for  by  increasing  the 
pressure.    Also  with  larger  openings  than  used  in  the  experiments, 
presumably  less  stress  difference  would  be  required  to  close  cavities. 
Under   the   conditions   of   the   experiment   cubical    compression 
played  an  important  part. 

A  factor  not  considered  in  the  above  estimates  is  the  fact  that 
under  tension  of  whatever  magnitude  the  rock  will  fracture  rather 
than  flow.  So  far  down  in  the  earth  as  tension  exists,  therefore, 
rock  fracture  may  extend. 

The  possibility  is  suggested  on  page  7  that  a  rock  may  yield 
to  compression  by  flowage — at  the  same  time  it  is  yielding  to 
tension  by  fracture.  If  this  is  possible,  the  fractures  are  really 
minor  and  subsidiary  to  the  flowage  and  therefore  require  only  a 
minor  modification  of  our  discussion  of  the  depth  at  which  a  rock 
will  flow. 

Estimates  of  the  depth  of  the  zone  of  rock  fracture  have  also 
been  made  by  studying  the  amount  of  erosion  necessary  to  uncover 
evidences  of  rock  flowage.  This  method,  by  its  very  nature,  must 
yield  indefinite  results;  and  yet,  as  applied  in  different  parts  of  the 
world  by  different  observers,  it  indicates  that  the  depths  below  the 
surface  necessary  for  rock  flowage  for  strong  rocks  are  possibly  a 
little  larger  than  those  derived  from  the  computations  of  Van  Hise 
and  Hoskins. 

Another  line  of  evidence  on  the  same  point  is  afforded  by  a 
study  of  earthquake  shocks.  Earthquakes  originate  in  the  frac- 
turing of  rocks,  and  in  no  case  has  their  point  of  origin  been  esti- 
mated to  be  more  than  nine  or  ten  miles  below  the  surface.  Also 
it  has  been  found  that  waves  traveling  along  a  chord  which  passes 
ten  or  twelve  miles  below  the  surface  at  the  deepest  point  are 

1  Adams,  Frank  D.,  An  experimental  contribution  to  the  question  of  the  depth 
of  the  zone  of  flow  in  the  earth's  crust:  Jour.  Geol.,  Vol.  20,  1912,  p.  115. 


DEPTH   OF   ROCK   FLOW  11 

sharply  discriminated  in  speed  and  in  position  of  their  planes  of 
vibration  from  waves  traveling  along  the  circumference.  Waves 
traveling  along  chords  at  shallower  depths  are  not  thus  easily 
differentiated.  Some  difference  in  medium  at  great  and  small 
depths  is  assuredly  indicated. 

Suggestive,  but  not  to  be  cited  as  definite  evidence,  is  the  fact 
that  the  mountains  of  the  earth's  crust  never  rise  much  above  five 
miles  in  height.  There  are  many  factors  which  control  summit 
levels.  One  of  them  has  been  suggested  to  be  the  yielding  of  the 
rocks  at  the  base  by  flowage  when  the  mountains  had  reached  a 
height  of  over  five  or  six  miles.  Prevalence  of  flowage  structures 
in  the  cores  of  mountains  are  in  accord  with  this  view,  though  many 
of  them  may  be  otherwise  explained. 

From  various  sources,  therefore,  there  is  evidence  or  suggestion 
that  the  zone  of  rock  fracture  is  comparatively  shallow,  perhaps 
less  than  twelve  miles  deep  for  the  strongest  rocks.  No  one  line 
of  evidence  cited  is  decisive.  Yet  there  is  such  accordance  of  the 
various  kinds  of  evidence  that  the  figures  above  given  may  be 
tentatively  accepted.  The  figures  may  be  increased  when  more  is 
known  of  the  ratio  of  rigidity  to  increase  of  depth. 

VOLUME  CHANGES  IN  FRACTURE  AND  FLOW 

Fracturing  itself  involves  increase  of  volume  of  the  fractured 
mass,  because  of  displacement  of  the  parts.  In  the  zone  of  fracture 
rocks  also  are  accessible  to  weathering  agencies  of  the  atmosphere 
and  hydrosphere  and  undergo  metamorphic  changes  which  increase 
their  volume.  Calculations  of  the  changes  of  volume  of  the  com- 
mon rocks  of  the  earth's  crust  indicate  a  maximum  increase  in 
volume  at  the  surface  of  50%  by  development  of  pore  space  and  of 
minerals  of  low  density.  In  the  zone  of  flow  there  is  a  tendency  to 
diminish  volume  by  closing  pore  space  and  by  developing  minerals 
of  higher  density. 

If  the  three  principal  stresses  are  equivalent,  the  rock  may  be 
cubically  compressed,  but  experimentally  no  permanent  compres- 
sion has  been  accomplished,  the  rock  expanding  as  soon  as  pressure 
is  released.  The  experiments  of  Adams1  show  that  acid  rocks  are 

1  Adams,  Frank  D.,  and  Coker,  Ernest  G.,  An  investigation  into  the  elastic  con- 
stants of  rocks,  more  especially  with  reference  to  cubic  compressibility:  Pub. 
No.  46,  Carnegie  Inst.  of  Wash.,  1906,  pp.  66-68. 


12  STRUCTURAL   GEOLOGY 

more  elastic  than  glass,  basic  rocks  less  so,  marbles  and  limestones 
about  the  same.  Of  the  minerals,  quartz  is  highly  elastic,  and 
therefore  he  concludes  that  the  high  elasticity  of  granite  is  probably 
due  to  this  cause. 


SURFACE  EXPRESSION  OF  THE  ZONES  OF  FRACTURE 

AND   FLOW 

Erosion  takes  advantage  of  fracture  planes  in  etching  the 
earth's  surface.  Where  rocks  are  homogeneous  and  the  fracture 
planes  are  in  well-defined  systems,  drainage  lines  may  be  in  more 
or  less  regular  patterns,  especially  in  non-glaciated  regions.  Where 
fractures  are  curved  and  discontinuous  and  not  in  regular  systems, 
this  may  be  represented  in  the  irregularity  of  the  erosion  channels. 
It  must  be  remembered  that  fractures  are  not  the  only  structures 
which  localize  erosion  channels.  Differing  resistance  of  rocks, 
bedding,  dip  of  impervious  layers,  etc.,  have  their  influence.  Hence 
it  should  not  be  assumed  that  all  drainage  patterns  correspond  to 
fracture  systems,  and  it  is  especially  unsafe  to  read  into  the  actual 
pattern  a  more  regular  pattern  based  on  a  hypothetical  conception 
of  fracture  systems. 

Dislocations  of  the  earth's  crust  may,  independently  of  erosion, 
cause  topographic  irregularities,  some  of  which  are  referred  to  in  a 
later  section  on  faults  (pp.  57-58) . 

Rocks  which  have  undergone  rock  flowage  are  for  the  most 
part  easily  eroded,  and  are  consequently  likely  to  be  relatively  low 
areas.  Schistosity  obliterates  expression  of  original  structures. 
The  schistose  structure  resulting  from  rock  flowage  may  give  linear 
elements  to  the  topography,  but  these  elements  are  likely  to  be 
curving,  overlapping,  discontinuous,  and  not  in  the  more  or  less 
regular  intersecting  sets  characteristic  of  rock  fracture. 

By  the  time  erosion  has  exposed  at  the  surface  rocks  which  have 
undergone  rock  flowage,  these  have  come  through  the  zone  of 
rock  fracture,  with  the  result  that  fractures  may  be  superposed 
upon  schistosity,  in  which  cases  the  surface  expression  may  com- 
bine features  characteristic  of  rock  flow  and  fracture. 

Other  things  being  equal,  evidence  of  flowage  is  likely  to  be 
more  conspicuous  in  areas  from  which  there  has  been  a  large 
amount  of  material  eroded  than  elsewhere.  The  rocks  showing  at 


SURFACE   EXPRESSION   OF  STRUCTURES          13 

the  surface  in  such  areas  have  been  buried  to  great  depths  below 
the  surface.  Older  rocks  are  likely  to  have  been  more  deeply 
buried  below  the  surface  than  younger  rocks,  and  therefore  to  have 
been  at  one  time  in  the  zone  of  rock  flowage,  but  this  does  not 
always  follow. 

It  is  frequently  possible  to  determine  from  the  study  of  geologic 
and  topographic  maps,  whether  the  rocks  of  an  area  are  characteris- 
tic of  the  zone  of  rock  flowage  or  rock  fracture.  Note  for  instance 
the  contrast  between  Archean  and  Algonkian  areas  in  most  parts 
of  the  Lake  Superior  region,  and  between  the  pre-Cambrian  and 
Paleozoic  areas  in  the  Piedmont  and  southern  Appalachians. 
The  student  may  study  to  advantage  the  evidences  of  fracture  and 
flow  on  the  following  maps  with  their  accompanying  sections: 

Roan  Mountain  folio,  Tennessee-North  Carolina,  No.  151,  U.  S.  G.  S. 
Pisgah  folio,  North  Carolina-South  Carolina,  No.  147,  U.  S.  G.  S. 
Gadsden  folio,  Alabama,  No.  35,  U.  S.  G.  S. 

Geology  of  the  Lake  Superior  Region,  Mon.  52,  U.  S.  G.  S.,  particularly 
maps  of  the  Marquette  and  Gogebic  districts. 

Note  the  areal  distribution  and  relations  of  rocks,  presence  of 
linear  elements,  evidences  of  thickening  or  thinning  by  flowage, 
schistosity,  drainage,  depth  to  which  rocks  have  been  covered,  etc. 
More  specific  expressions  of  the  zones  of  fracture  and  flow  at  the 
rock  surface  are  discussed  on  later  pages. 


FRACTURES 

Rock  fractures  are  usually  designated  by  terms  such  as  joints, 
faults,  fracture  cleavage,  autoclastic  structures,  etc.  The  variety 
of  names  and  classifications  of  rock  fractures  should  not  obscure 
the  fact  that  after  all  they  are  only  expressions  of  the  ordinary 
mechanical  principles  of  the  breaking  of  solid  materials.  We 
may  for  the  time  avoid  some  complexity  of  names,  therefore,  by 
outlining  first  some  of  the  simpler  mechanical  features  of  the  frac- 
turing of  rocks,  applicable  to  all  rock  fractures  regardless  of  names. 
To  do  this  adequately  would  require  the  use  of  many  of  the  techni- 
cal terms  of  mechanics,  which,  for  the  purpose  of  this  volume, 
would  be  undesirable.  In  the  following  account  of  the  principles 
of  fracturing  the  attempt  is  made  to  use  non-technical  language, 
even  though  this  may  seem  to  the  technical  reader  to  be  at  the 
expense  of  accuracy  and  conciseness.  Some  technical  terms  are 
unavoidable. 

ATTITUDES    OF   FRACTURES    WITH    REFERENCE   TO 

STRESSES 

Stress  is  defined  as  the  reaction  of  the  interior  parts  of  a  solid 
against  forces  tending  to  deform  it,  and  strain  is  the  change  in 
shape  of  the  solid  resulting  from  these  reactions.  All  stresses  act- 
ing at  any  point  may  be  resolved  into  three  mutually  perpendicular 
components  or  principal  axes  of  stress.  There  are  correspondingly 
principal  axes  of  strain. 

Fractures  form  in  the  following  relations  to  stress : 

TENSION   FRACTURES 

Under  tension,  fractures  tend  to  develop  in  planes  normal  to 
the  maximum  stress.  There  are  also  shearing  stresses  inclined  to 
the  maximum  tension,  just  as  there  are  in  compression  (see  p.  16), 
but  only  rarely  does  the  breaking  of  the  rock  mass  follow  these 

14 


TENSION   FRACTURES 


15 


planes  of  shearing  stress,  because  the  resistance  of  the  rock  to 
tension  is  less  than  its  resistance  to  shearing. 

Tension  fractures  may  develop  also  when  a  mass  is  deformed  by 
shearing  in  the  manner  described  on  page  16. 

By  torsion,  intersecting  sets  of  fractures  have  been  simulta- 
neously produced  at  angles  of  45°  to  the  axis  of  torsion.  These 


FIG.  2.  Diagram  to  illustrate  the  development  of  rectangular  sets  of  tension  frac- 
tures under  torsion.    After  Daubree. 


fractures  are  probably  due  to  tension  rather  than  compression.  If  a 
circle  be  drawn  on  the  flat  side  of  a  rubber  eraser  and  the  eraser 
twisted  it  will  be  noted  that  the  elongation  of  this  circle,  indicating 
tension,  is  normal  to  the  planes  followed  by  fracture  in  torsion 
tests1  (Fig.  2). 

1  Becker,  G.  F.,  The  torsional  theory  of  joints:  Trans.  Am.  Inst.  M.  E.,  Vol.  24, 
1895,  p.  136. 


16  STRUCTURAL  GEOLOGY 


COMPRESSION  FRACTURES 

Under  compressive  stresses,  fractures  tend  to  develop  along 
planes  of  " maximum  shear,"  which  are  inclined  to  the  direction  of 
principal  stresses;  but  the  degree  of  inclination  and  the  direction  of 
dip  of  the  planes  away  from  the  direction  of  maximum  stress  vary 
between  the  following  limiting  cases : 

A  building  stone  cube,  subjected  to  pressure  on  one  pair  of 
opposite  sides,  shears  in  planes  45°  or  less  from  the  line  of  maximum 
pressure.  These  planes  may  dip  in  one  direction  away  from  this 
line  or  may  dip  outward  in  all  directions,  developing  a  cone.  If  the 
cube  be  subjected  to  pressure  as  before,  while  it  is  being  rigidly  sup- 
ported on  another  pair  of  opposite  sides,  the  remaining  surface 
being  free,  fractures  will  develop  dipping  toward  the  free  sides. 
Portions  of  the  rock  mass  will  thus  be  displaced  in  the  direction 
of  these  free  sides.  This  presumably  is  a  common  case  in  nature, 
as,  for  instance,  where  a  horizontal  stress  affecting  a  homogeneous 
rock  mass  is  relieved  principally  by  displacement  upward.  The 
planes  of  fracture  dip  from  the  surface  toward  the  greatest  com- 
pression and  displacement  along  these  planes  will  carry  the  rock 
mass  upward,  in  the  manner  of  a  thrust  fault. 

The  compressive  strains  thus  far  described  are  known  as  non- 
rotational;  l  that  is,  the  principal  directions  of  stress  remain  constant 
with  reference  to  the  principal  axes  of  strain  throughout  the  defor- 
mation. Fully  as  common  in  nature  are  rotational  strains  or 
shears,  in  which  the  strain  axes  are  being  constantly  rotated  during 
the  deformation,  illustrated  by  Fig.  7.  The  fractures  are  then  not 
symmetrically  grouped  with  reference  to  the  principal  stress  but 
they  retain  much  the  same  relations  to  the  elongation  and  short- 
ening of  the  deformed  mass,  as  in  the  case  of  non-rotational  strain 
above  described.  The  principal  stress  usually  intersects  the  obtuse 
angle  between  such  fractures. 

One  of  the  incidental  accompaniments  of  fracture  by  shearing 
under  a  rotational  compressional  stress  may  be  development  of 
tension  fractures  in  planes  normal  to  the  elongation  of  the  mass. 

A  convenient  way  to  remember  and  picture  the  system  of  frac- 
tures developed  under  the  above  stress-strain  relations  is  by 

iHoskins,  L.  M.,  Flow  and  fracture  of  rocks  as  related  to  structure:  16th 
Ann.  Kept,  U.  S.  Geol.  Survey,  pt.  1,  1896,  p.  845  et  seq. 


COMPRESSION   FRACTURES 


17 


FIG.  3.  Results  of  crushing  wooden  blocks  by  non-rotational  strain.  Note  ten- 
dency of  fractures  to  follow  shearing  planes  45°  to  the  pressure  (which  was  from 
above)  regardless  of  the  grain  of  the  wood. 


FIG.  4.  Fracture  of  building  stone  (brown  sandstone)  along  shearing  planes. 

Buckley. 


After 


18 


STRUCTURAL   GEOLOGY 


FIG.  5.  Wire  netting  model  undeformed.    See  also  Figs.  6  and  7. 

the  use  of  the  sphere  as  the  unit  of  original  structure  and  the  strain 
ellipsoid  as  its  deformed  equivalent.  Fractures  under  compression 
tend  to  follow  the  cross  sections  in  the  strain  ellipsoid  which  are  the 
same  in  dimensions  as  those  of  the  original  sphere;  in  other  words, 
planes  (called  planes  of  no  distortion)  determined  by  the  intersec- 
tions of  the  original  sphere  with  the  strain  ellipsoid. 


COMPRESSION   FRACTURES 


19 


A  simple  device  for  illustrating  the  position  of  strain  ellipsoid 
and  shearing  planes  in  both  rotational  and  non-rotational  strain 
is  shown  in  Figs.  5,  6,  and  7.  A  cardboard  upon  which  is  inscribed 
a  circle  is  laid  between  two  sheets  of  wire  netting.  The  three  are 
then  fastened  together  by  a  rivet  in  the  center  of  the  circle.  A 
wooden  hinged  frame  fastened  to  the  netting  allows  and  controls 
the  distortion  of  the  netting,  while  the  interior  sheet  remains  undis- 


FIG.  6.  Wire  netting  model  deformed  by  non-rotational  strain.  Straight  lines 
connecting  intersections  of  circle  and  ellipse  mark  positions  of  "planes  of  no 
distortion"  or  planes  of  maximum  shear. 

torted.  A  circle  and  diameters  are  painted  on  the  netting  corre- 
sponding with  those  on  the  central  sheet.  When  the  screen  is 
distorted  the  circle  on  the  wire  becomes  an  ellipse  or  a  cross  section 
through  the  greatest  and  least  principal  axes  of  a  " strain  ellipsoid," 
which  is  superposed  upon  the  undeformed  circle  of  the  cardboard. 
In  Fig.  6  a  non-rotational  strain  is  represented,  called  "pure 
shortening  and  elongation."  The  circle  elongates  normal  to  the 
pressure.  The  planes  of  no  distortion,  which  are  the  planes  of 


20 


STRUCTURAL   GEOLOGY 


maximum  shear,  stand  normal  to  the  surface  of  the  screen.  Their 
intersections  with  the  plane  of  the  screen  are  to  be  seen  at  about 
45°  to  the  pressure.  It  will  be  noted  that  the  lines  representing  the 
planes  of  shear  are  parallel  to  the  wires.  The  distortion  of  the 
screen  actually  occurs  by  shearing  of  the  wire  mesh.  This  should 


FIG.  7.  Wire  netting  model  deformed  by  rotational  strain,  or  shear.  Straight  lines 
connecting  intersections  of  circle  and  ellipse  mark  positions  of  "planes  of  no 
distortion"  or  planes  of  maximum  shear. 

make  clear  the  fact  that  the  painted  lines  of  "no  distortion"  are 
actually  shearing  planes. 

In  Fig.  7  the  strain  is  a  rotational  one.  A  strain  ellipse  is  pro- 
duced by  shearing  of  the  top  over  the  bottom  of  the  model,  ob- 
viously by  movement  along  the  shearing  planes  of  the  wire  mesh. 


COMPRESSION   FRACTURES  21 

The  planes  of  no  distortion  are  indicated  as  before.  It  will  be  noted 
that  they  have  the  same  relations  to  the  ellipse  as  before,  though 
the  pressure  has  been  applied  at  a  different  angle.  It  is  evident 
that  the  net  result  is  the  same  as  in  a  non-rotational  strain,  so  far 
as  the  shape  of  the  strain  ellipse  is  concerned.  The  rotation  of  the 
one  figure  in  space  would  make  it  coincide  with  the  other. 

It  cannot  be  too  strongly  emphasized  that  in  nature  what  we 
usually  see  is  the  net  result,  which  we  may  interpret  in  terms  of 
strain  ellipsoid.  This  strain  ellipsoid  may  have  been  developed 
either  by  rotational  or  non-rotational  strain,  and  we  must  be  care- 
ful not  to  assign  the  strain  ellipsoid  to  either  kind  of  strain  on 
insufficient  evidence.  There  are  cases  where  it  is  possible  to  make 
such  definite  assignment. 

Rock  fracture  tends  to  occur  under  any  one  of  the  stress-strain 
relations,  or  some  combination  of  them,  described  above  under  the 
headings  Tension  Fractures  and  Compression  Fractures.  Initial 
planes  of  weakness  may  modify  these  relations.  In  homogeneous 
masses  these  are  the  limiting  cases  which  cover  all  rock  fractures. 
In  the  field  study  of  fracture  it  is  sometimes  possible  to  determine 
what  the  stress-strain  relations  have  been;  commonly  it  is  not.  It 
seems  to  the  writer,  therefore,  that  great  care  should  be  taken  in 
choosing  a  general  nomenclature  for  fractures  wilich  would  not 
imply  a  knowledge  of  stress-strain  relations  we  do  not  possess. 

JOINTS 

It  is  sometimes  convenient  to  classify  joints  as  strike  joints  or 
dip  joints,  to  indicate  concisely  their  parallelism  in  direction  with 
the  strike  or  dip  of  beds.  Joints  are  ordinarily  classified  as  tension 
and  compression  joints  to  express  their  relations  to  stresses.  In 
nine  cases  out  of  ten  the  student  sees  nothing  in  the  joint  itself 
which  tells  him  whether  the  joint  results  from  tension  or  compres- 
sion, and  the  attempt  to  use  this  classification  may  lead  to  un- 
warranted conjecture,  or  may  throw  him  into  the  discouraged 
state  of  mind  of  a  person  who  believes  that  he  should  be  able  to 
tell  something  which  the  facts  do  not  readily  indicate.  It  is 
pertinent  to  inquire  as  to  what  conditions  tell  definitely  whether 
any  particular  system  of  joints  is  due  to  tensional  or  compressive 
stress. 


22  STRUCTURAL   GEOLOGY 

JOINTS   WHICH   CAN   BE  CLASSIFIED  AS   DUE   TO  TENSION 

(a)  Faulting  may  imply  extension  of  surface  (see  pp.  55-56), 
and  hence  the  association  of  joints  with  such  faulting  would 
suggest  their  development  by  tensional  stresses. 

(b)  Open  joints  indicate  tension,  but  it  is  difficult  to  determine 
whether  tension  existed  at  the  time  the  joints  were  formed  or  was 
subsequent  to  their  genesis. 

(c)  Tension  joints  have  been  found  along  the  crests  of  anti- 
clines, developed  as  indicated  in  the  diagram  (Figs.  8  and  53). 
These,  however,  are  usually  on  a  small  scale.    The  writer  knows  of 


FIG.  8.  Tension  joints  on  anticline.    After  Van  Hise. 

no  cases  described  for  the  United  States  in  which  any  regional  set 
of  joints  has  been  positively  related  to  tensional  stresses  developed 
along  major  anticlines,  but  the  existence  of  such  cases  is  reasonably 
inferred  where  joints  are  parallel  to  the  axial  planes  of  folds. 

(d)  During  the  process  of  cooling  in  igneous  rocks,  tensional 
stresses  are  set  up  in  them;  and  these  stresses  result  in  the  forma- 
tion of  joints,  not  only  in  the  igneous  masses  themselves,  but  in  the 
adjacent  rocks.  The  remarkably  complicated  fractures  of  Tonopah 
and  other  mineral-bearing  districts  of  the  Great  Basin  first  sug- 
gested this  origin,  and  it  seems  to  be  now  an  established  fact  that 
much  of  the  complex  fracturing  of  igneous  rocks  may  be  related 
definitely  to  their  cooling.  (See  page  43.)  Such  joints  may  not 
be  persistent  or  in  regular  systems.  Locally  the  fractures  take 
certain  curved  or  concentric  forms  about  loci  of  cooling,  as  for 
instance,  in  the  gabbro  of  the  Cobalt  district  of  Ontario,  or  in  the 
slates  with  which  the  gabbro  has  come  into  contact.  These  slates 
have  been  heated  and  caused  to  expand  under  the  influence  of  the 
intrusive  and  have  subsequently  cracked  on  loss  of  heat.  Basaltic 
parting  is  only  a  special  type  of  tension  jointing  developed  by 


TENSION   JOINTS  23 

i 

cooling.  Radial  and  peripheral  fractures  seem  in  some  cases  to 
have  been  developed  by  the  cooling  of  laccoliths  and  batholiths. 
Laccoliths  have  sometimes  been  supposed  to  pull  away  from  the 
walls  in  the  manner  of  a  cooling  melt  from  a  mold,  as  for  instance, 
in  the  Iron  Springs  district  of  Utah. 

(e)  Another  type  of  local  tension  jointing  is  developed  by  the 
drying  out  of  a  sediment,  resulting  in  the  formation  of  mud  cracks; 
or  the  desiccation  of  sediments  on  a  large  scale.     The  joints  so 
formed  lack  regularity  and  persistence.    It  is  possible  that  many  of 
the  fairly  extensive  joints  in  flat-lying  sedimentary  beds  like  the 
Paleozoic  of  the  Mississippi  valley  may  be  due  to  the  drying  and 
settling  of  the  formation. 

(f)  In  some  cases  where  dominant  joints  can  be  identified  as  the 
result  of  shearing  stresses,  as  for  instance,  in  a  shaly  layer  sheared 
between  two  hard  quartzite  beds,  small  tension  gash  joints  have 
been  an  incidental  development.      (See  pp.  16  and  25). 


JOINTS  WHICH  CAN  BE  CLASSIFIED  AS  DUE  TO  COMPRESSION 

(a)  Compressive  joints  may  be  sometimes  identified  by  evi- 
dences of  slipping,  such  as  slickensides,  developed  along  the  joint- 
ing planes;  but  these  evidences  do  not  necessarily  indicate  that  the 
compressive  stresses  were  applied  at  the  time  the  joints  were 
formed. 

(b)  Where  these  joints  pass  into  overthrust  faults  or  folds,  as, 
for  instance,  in  the  southern  Appalachians,  they  are  likely  to  be 
compression  joints. 

(c)  Compressive  joints  may  also  be  identified  frequently  on  the 
limbs  of  folds  by  the  manner  in  which  they  follow  closely  the 
theoretical  directions  required  for  compressive  shear  by  the  stress 
conditions  occurring  at  those  places  (see  pp.  20-21).    For  instance 
in  the  Baraboo  quartzite  in  Wisconsin  (see  Figs.  9  and  11),  there 
are  joints  parallel  to  the  bedding,  along  which  there  has  been  a 
slight  amount  of  slipping;  there  is  another  set  inclined  to  the  bed- 
ding; this  latter  set  is  continuous  in  direction  only  through  homo- 
geneous beds  and  passes  to  other  beds  by  an  offset  or  a  curve  along 
the  bedding  planes.     In  the  softer  beds  the  joints  are  so  closely 
spaced  as  to  yield  a  "fracture  cleavage"  (see  p.  63).    The  joints 
have  positions  accordant  with  the  supposition  that  they  have  been 


FIG.  9.  Fracture  cleavage  and  jointing  developed  by  shearing  between  beds  in  Baraboo 
quartzite.  After  Atwood.  The  light  portion  on  the  right  is  a  bed  of  brittle  quartz- 
ite.  The  dark  portion  on  the  left  is  a  bed  of  softer  shaly  quartzite.  The  outcrop  is 
a  part  of  the  north  limb  of  a  syncline.  The  right  hand  bed  is  on  the  south.  It  has 
obviously  moved  upward  with  reference  to  the  beds  to  the  north  of  it,  as  would  be 
expected  from  this  position  on  the  syncline.  The  fractures  here  have  been  de- 
veloped by  rotational  or  shearing  stresses  described  on  pp.  16,  20.  It  is  suggested 
that  the  student  superpose  on  these  beds  the  theoretical  positions  of  the  strain  el- 
lipsoids and  the  planes  of  maximum  shear.  Note  relations  of  fracture  cleavage  to 
jointing  in  adjacent  bed.  (See  also  page  121). 

24 


COMPRESSION   JOINTS 


25 


formed  by  compressional  shearing,  caused  by  slipping  between  the 
beds.  Short  open  gashes  or  joints  are  also  developed  here  by  ten- 
sion, as  indicated  in  the  figure. 

(d)  The  sheet  structure  so  commonly  observed  and  utilized  in 
granite  and  other  quarries  is  a  system  of  jointing  probably  at  least 
in  part  developed  under  compressive  stresses.  (Figs.  12,  13  and  14.) 


FIG.  10.  Fracture  cleavage  developed  in  slaty  quartzite  layer  between  two  massive 
beds  of  quartzite,  on  south  limb  of  the  Baraboo  syncline,  Wisconsin.  Note  the 
direction  of  differential  movement  and  correlate  this  with  position  on  the  fold. 
What  are  the  relations  of  the  cleavage  to  pressure?  Note  relations  of  fract- 
ure cleavage  to  joints  in  the  adjacent  massive  layers.  (See  also  Fig.  37  and 
page  121). 

The  sheets  are  thinnest  near  the  surface  and  rapidly  thicken  below. 
They  may  be  curved,  and  in  general  are  parallel  with  the  rock  sur- 
face. Usually  they  are  found  to  be  lens-shaped  when  traced  some 
distance.  Many  instances  have  been  noted  of  a  lengthening  of 
blocks  when  quarried  out,  sometimes  with  explosive  violence, 
indicating  that  in  the  ledge  they  were  under  compressive  stress. 


26 


STRUCTURAL   GEOLOGY 


Compression  is  indicated  also  by  the  occasional  flattening,  by 
faulting,  of  drill  holes  and  other  openings.1  The  sheet  structure  is 
developed  artificially  by  the  use  of  explosives,  by  hot  air,  and  by 
heating  the  surface.  These  compressive  stresses  have  been  re- 
ferred to  various  causes — solar  heat,  weathering  (or  kaoliniza- 
tion),  expansion  of  the  surface  due  to  removal  of  overlying  load 
by  erosion,  and  to  major  earth  movements.2 

>    South 


FIG.  11.  Vertical  section  Baraboo  quartzite,  normal  to  the  strike,  on  the  South 
Range,  Baraboo  district,  Wisconsin,  showing  joints  formed  by  the  folding  of 
weak,  thin  beds  interstratified  with  thick,  strong  beds.  After  Steidtmann. 
Short  open  gashes  or  tension  joints  may  be  seen  crossing  the  curved  compres- 
sion joints  in  the  softer  layers. 

Whatever  the  cause,  the  upper  layers  tend  to  extend  themselves 
farther  than  the  lower  layers  by  shearing,  producing  the  sheeting 
planes  between  them.  The  same  structure  has  been  referred  also 
to  tension  due  to  cooling  of  the  igneous  rocks  while  still  under 
sedimentary  load,  the  sheets  being  approximately  parallel  to  the 

1  Dale,  T.  Nelson,  The  granites  of  Vermont:  Bull.  404  U.  S.  G.  S.,  1909,  pp.  17-18. 
2 Idem;  also  Bulls.  354  and  484,  U.  S.  Geol.  Survey. 


COMPRESSION   JOINTS 


27 


original  contact  surface  of  the  intrusive.  Bearing  in  mind  the 
parallelism  of  the  sheets  to  the  present  erosion  surface,  and  their 
diminution  in  number  below  the  surface,  the  explanation  of  tension 
by  cooling  involves  the  assumption  that  the  present  erosion  sur- 


FIG.  12.  Sheet  structure  in  granite.    After  Dale. 

face  is  nearly  the  same  as  the  original  contact  surface,  which  cer- 
tainly is  not  always  true. 

The  sheets  are  crossed  by  vertical  joints  which  partly  result  from 
tension  due  to  gravity  acting  on  the  thin  sheets.  Some  of  them  also 
may  be  compressive.  By  application  of  the  principles  of  breaking 
under  rotational  or  shearing  strain  given  above,  it  will  appear 
that  a  complementary  set  of  compression  fractures  should  be 


28 


STRUCTURAL   GEOLOGY 


expected  approximately  at  right  angles  to  the  sheeting  planes. 
In  quarries  these  vertical  joints  may  be  in  one  or  more  intersecting 
sets.  They  are  characteristically  intermittent,  extending  through 
a  given  set  of  sheets  and  offsetting  in  the  sheets  above  and  below. 
Not  infrequently  they  are  curved. 


fa-) 


(bi 


FIG.  13.  Spalling  of  surface  by  shearing  due  to  heating  or  cooling.  After  Van  Hise 
(a)  shows  the  condition  of  a  block  of  uniform  temperature,  (b)  illustrates  the 
manner  in  which  the  upper  portion  of  a  rock  surface  expands  when  heated 
above  average  temperature;  where  the  difference  in  temperature  is  sufficiently 
great,  this  results  in  the  splitting  off  of  the  upper  layers,  (c)  illustrates  the 
contraction  of  the  upper  surface  by  cooling  below  the  average  temperature; 
where  the  difference  in  temperature  is  sufficiently  great,  this  results  in  the 
splitting  off  of  the  upper  layers. 

JOINTS  DEVELOPED  UNDER  UNKNOWN  STRESS-STRAIN 
CONDITIONS 

Probably  the  great  majority  of  joints  has  not  yet  been  satisfac- 
torily determined  as  belonging  to  the  tension  or  compression  class. 
Many  instances  might  be  cited  of  attempted  classification  without 


JOINTS   WIDENED   BY   GROWING   CRYSTALS      29 

sufficient  proof.  Especially  numerous  have  been  the  attempts 
to  classify  joints  as  compressive  when  they  are  in  vertical  inter- 
secting sets,  on  the  assumption  that  the  intersection  of  the  joints  is 
an  indication  of  compression  stresses.  On  the  other  hand,  identi- 
cally similar  sets  of  joints  have  been  referred  to  tension  acting  in 
mutual  perpendicular  directions,  or  to  torsion  in  the  manner 
indicated  by  Daubree's  experiment.  (See  p.  15.) 

WIDENING  OF  JOINTS  BY  THE  LINEAR  FORCE  OF  GROWING 

CRYSTALS 

It  has  long  been  known  that  crystals  exert  very  considerable 
force  in  growing.  Crystals  of  pyrite,  for  instance,  drive  apart  the 
laminse  of  slates.  Experiments  on  the  pressure  exerted  by  growing 
crystals  of  alum  and  other  salts  have  shown  that  they  exert  a 
pressure  of  the  same  order  of  magnitude  as  the  ascertained  resist- 
ance which  the  crystals  offer  to  crushing  stresses.1  It  is  supposed 
that  this  force  exerted  by  crystals  may  be  a  factor  in  widening 
mineral-filled  fissures,  like  the  gold-bearing  quartz  veins  of  the 
Mother  Lode  of  California,  some  of  which  have  a  width  of  several 
hundred  feet.  This  width  is  not  observed  in  unfilled  fissures. 
In  fact,  the  unfilled  fissures  are  in  general  very  narrow  as  com- 
pared with  the  fissures  which  have  been  filled  and  cemented. 
According  to  Becker,2  laminse  of  the  slates  on  two  sides  of  Mother 
Lode  veins  have  locally  been  driven  apart  and  contorted.  He  con- 
cludes that  when  such  occurrences  cannot  be  accounted  for  by 
faulting  the  inference  is  almost  unavoidable  that  the  laminse  have 
been  driven  apart  by  the  force  of  growing  crystals  of  quartz,  the 
axes  of  which  stand  sensibly  at  right  angles  to  the  planes  of  the 
laminse.  The  ribbon  ore,  consisting  of  parallel  laminse  of  slate, 
separated  by  quartz,  has  been  regarded  as  due  to  faulting,  but 
evidence  of  faulting  is  often  lacking  and  it  is  difficult  to  conceive 
how  faulting  could  separate  these  slate  bands  so  evenly.  Separa- 
tion by  the  growing  force  of  quartz  crystals  is  an  alternative  ex- 
planation. 

If  quartz  during  crystallization  exerts  a  pressure  on  the  sides 
of  the  vein  which  is  of  the  same  order  of  magnitude  as  the  resist- 

1  Becker,  G.  F.,  and  Day,  Arthur  L.,  The  linear  force  of  growing  crystals:  Proc. 
Wash.  Acad.  Sci.,  Vol.  7,  1905,  pp.  283-288. 

2  Op.  cit.,  p.  284. 


30 


STRUCTURAL  GEOLOGY 


ance  which  it  offers  to  crushing,  as  Becker1  thinks  probable,  then 
this  force  is  also  of  the  same  order  of  magnitude  as  the  resistance 
of  wall  rocks,  and  thus  it  becomes  possible  that  the  widening  of  the 
filled  fissures  may  be  largely  due  to  this  cause. 

SURFACE  EXPRESSION  OF  JOINTS 

What  has  already  been  written  about  the  surface  expression 
of  the  zone  of  fracture  applies  specifically  to  joints.  One  need  only 
cite  the  Grand  Canyon,  Yosemite  Valley,  or  the  Dells  of  the  Wis- 


FIG.   14.  Spalling  of  andesite  outcrops,  presumably  due  to  alternate  heating  and 
cooling  in  weathering. 


consin  river,  where  the  drainage  has  been  controlled  almost  en- 
tirely by  joints.  In  other  areas,  especially  in  drift-covered  areas, 
the  relation  may  be  a  very  slight  one.  In  some  cases  the  assump- 
tion of  relationship  has  been  carried  so  far  that  drainage  lines  have 
been  taken  as  evidence  of  joints  without  further  information,  and 
continuity  and  regularity  of  joint  systems  have  been  assumed  on  a 
basis  of  too  little  information.  Attention  has  been  called  above  to 
joints  of  wide  distribution  which  characteristically  lack  regularity. 

1  Op.  cit.,  p.  287. 


LABORATORY  STUDY   OF  JOINTS  31 

SUGGESTIONS   FOR  LABORATORY  WORK   ON   JOINTS 
(See  also  pp.  60-61) 

On  the  experimental  side  the  suggestions  made  in  connection  with 
faults  on  pages  60-61  apply  equally  well  to  joints.  Much  can  be  done 
with  maps  of  joints.  In  this  connection  it  is  to  be  remembered  that  it  is 
the  interpretation  of  joints  that  is  wanted  and  not  a  mere  description  of 
joints. 

It  is  suggested  that  the  student  study  the  joints  of  the  areas  named 
below  and  make  the  attempt  to  classify  them  as  tensional  or  compres- 
sional;  and  if  compressional  to  discriminate  between  rotational  and  non- 
rotational  strains.  He  should  not  go  farther  in  inferences  than  the  facts 
warrant.  If  he  becomes  satisfied  of  the  origin  of  certain  joints  he  should 
not  assume  that  all  joints  in  this  area  have  the  same  origin.  Inferences 
from  the  facts  should  be  drawn  regardless  of  what  is  said  about  the  joints 
in  the  accompanying  reports. 

"Joint  system  in  the  rocks  of  southwestern  Wisconsin  and  its  relation  to  the 
drainage  network"  by  Edmund  Cecil  Harder,  Bulletin  of  University  of  Wis- 
consin, Science  Series,  Vol.  3,  No.  5.  The  joints  here  described  are  fairly 
typical  of  the  joints  of  the  flat-lying  Paleozoic  beds  of  the  Mississippi 
valley.  Careful  reasoning  from  the  facts  will  eliminate  certain  hypotheses 
of  the  origin  of  these  joints  and  point  with  a  reasonable  certainty  to  the 
true  origin.  Somewhat  similar  conditions  in  the  Grand  Canyon  and 
Yosemite  Valley  should  be  studied;  also  the  Watrous,  New  Mexico,  topo- 
graphic map.  The  relation  between  topography  and  jointing,  due  to 
interaction  of  climate,  rock  structure,  and  lithology,  is  to  be  noted. 

"The  secondary  structures  of  the  eastern  part  of  the  Baraboo  quartzite 
range,  Wisconsin"  by  Edward  Steidtmann,  Journal  of  Geology,  Vol.  XVIII, 
No.  3,  1910.  The  problems  of  jointing  in  folded  rocks  are  here  illustrated 
and  discriminated  with  unusual  clearness. 

"Granites  of  Maine,  Massachusetts,  New  Hampshire,  Rhode  Island,  and 
Vermont"  by  T.  Nelson  Dale,  Bulletins  313,  354,  404,  U.  S.  G.  S.  The 
jointing  of  igneous  rocks  is  here  admirably  illustrated  and  discussed. 
Before  reading  Dale's  discussion  of  origin,  the  facts  of  jointing  which  he 
describes  should  be  carefully  considered  and  an  attempt  made  to  formulate 
a  reasonable  hypothesis  of  origin  to  fit  these  facts.  Then  compare  with 
Dale's  conclusion. 

FAULTS 

Faults  are  fractures  along  which  there  has  been  some  relative 
displacement  of  the  rocks.  They  differ  from  joints  mainly  in  the 
extent  of  the  displacement  and  in  the  emphasis  on  the  displacement 
parallel  to  the  plane  of  fracture  rather  than  normal  to  it.  All 
fractures  are  accompanied  by  some  displacement — in  fact,  frac- 


32  STRUCTURAL   GEOLOGY 

tures  would  not  occur  were  not  some  displacement  required  by  the 
stresses. 

NOMENCLATURE 

The  elements  of  a  fault  are:  hade,  or  the  angle  made  by  the 
fault  plane  with  the  vertical;  dip,  or  the  angle  made  by  the  fault 
plane  with  the  horizontal;  throw,  or  the  displacement  of  the  beds 
measured  parallel  to  the  dip  of  the  fault  plane;  and  the  heave  or 
shift,  or  the  displacement  of  the  masses  measured  parallel  to  the 
strike  of  the  fault  plane.  When  a  fault  plane  dips  toward  the 
downthrow  side,  the  fault  is  called  a  normal  or  gravity  fault 
(Fig.  17).  The  displacement  of  the  crust  by  such  faults  is  ap- 
parently downward  and  therefore  apparently  due  to  gravitational 
forces.  Where  the  fault  plane  dips  toward  the  upthrow  side  of  the 
fault,  the  fault  is  called  a  reverse  or  thrust  fault  (Fig.  18).  The  dis- 
placement of  the  crust  is  then  apparently  of  the  nature  of  tangen- 
tial shortening.  Normal  faults  may  result  in  the  dropping  of 
blocks  called  graben.  These  usually  have  polygonal  outlines. 
Blocks  standing  up  between  graben  are  called  horsts  or  bridges. 
A  fault  with  vertical  displacement  is  expressed  at  the  surface  as  a 
small  cliff  or  scarp  to  which  the  name  "  fault  scarp"  has  been  given. 
" Fault  trace,"  " furrow,"  and  "rift"  are  terms  given  to  the  line  of 
intersection  of  the  fault  plane  with  the  surface.  They  are  es- 
pecially used  where  the  fault  displacement  is  horizontal  and  there 
is  no  fault  scarp,  or  where  the  fault  scarp  has  been  worn  down  by 
erosion. 

It  will  be  noted  that  the  classification  of  faults  into  normal  or 
gravity  and  reverse  or  thrust  faults,  takes  account  only  of  apparent 
relative  displacement  in  a  vertical  section  normal  to  the  fault  plane. 
It  takes  no  account  of  horizontal  or  oblique  displacement.  It 
expresses  merely  the  present  relations  of  the  beds  in  a  two  dimen- 
sional cross  section  rather  than  in  three  dimensions.  It  tells  us 
nothing  of  the  actual  displacements  of  the  beds.  Hinge  or  pivotal 
faulting  about  an  axis  normal  to  the  plane  of  faulting  may  produce 
a  fault  which  on  one  side  of  the  pivotal  axis  would  be  called  normal 
and  on  the  other  side  reverse,  and  yet  there  may  not  be  any  differ- 
ential movements  in  the  centers  of  the  mass  of  the  two  parts  of 
the  faulted  body  (Fig.  23).  A  purely  horizontal  displacement  may 
appear  either  as  a  normal  or  reverse  fault  at  any  one  place,  de- 


NOMENCLATURE  OF  FAULTS 


33 


FIG.  15.  Perspective  view  and  vertical  section  of  a  thrust  fault.    After  Willis. 


FIG.  16.  Diagram  of  a  thrust  fault.    After  Willis. 


34 


STRUCTURAL  GEOLOGY 


pending  upon  the  attitude  of  the  beds  with  regard  to  the  plane  of 
the  fault  (Figs.  19,  20,  21,  22,  24,  25). 


FIG.  17.  To  illustrate  relative  positions  of  blocks  in  normal  or  gravity  faulting. 

In  general  we  have  attempted  to  use  too  simple  a  nomenclature 
by  which  to  classify  faults.  The  classification  is  inadequate  to 
give  any  accurate  description  of  the  great  variety  of  relative  dis- 


FIG.  18.  To  illustrate  relative  positions  of  blocks  in  thrust  or  reverse  faulting. 

placements  possible  along  a  fault  plane.  The  inadequacy  of 
the  old  method  has  been  realized  in  recent  years  by  many  workers 
in  the  field,  especially  by  men  who  have  found  it  necessary  to  work 


NOMENCLATURE   OF   FAULTS  35 


FIG.  19.  Normal  faulting  produced  by  horizontal  movement  along  table  top. 


FIG.  20.  Reverse  or  thrust  faulting  produced  by  horizontal  movement  along 

table  top. 


36  STRUCTURAL   GEOLOGY 

out  in  detail  the  extremely  complicated  fault  systems  in  rocks 
associated  with  certain  ore  deposits.  Consequently  there  have 
been  several  attempts  to  develop  a  more  adequate  nomenclature 
to  describe  the  great  variety  of  conditions  met  with  in  the  field. 
Some  of  these  attempts  are  very  elaborate.  There  is  yet  no  general 
agreement  on  any  of  these  schedules.1  Therefore  none  of  these 
classifications  will  be  given  here.  It  may  be  questioned  whether 
special  nomenclature  of  faults  is  necessary.  There  has  been 
developed  in  mechanics  a  technical  nomenclature  to  describe  dis- 
placements of  all  solid  substances,  which  may  as  well  be  used  for 
faults  as  a  long  series  of  names,  difficult  to  remember,  coined  for 
the  special  use  of  the  geologist.  Quoting  from  Chamberlin  2 : 
The  complaint  "that  our  predecessors  have  trammelled  us  with 
premature  and  ill-chosen  classes  and  names  has  for  its  logical 
response  a  forbearance  on  our  part  from  further  imposition  of  the 
kind  on  our  successors ;  perhaps  also  it  suggests  an  effort  to  free  our- 
selves from  our  hamperings  by  dropping  embarrassing  terms,  and 
by  de-technicalizing  such  as  it  seems  best  to  retain." 

The  terms  "  normal  fault"  and  "  thrust  or  reverse  fault"  have 
become  so  well  intrenched  in  the  literature  of  the  subject  that  it  is 
difficult  to  avoid  their  use.  The  writer  believes  that  the  two  terms 
should  be  retained  to  express  apparent  displacements  in  a  plane  of 
section  normal  to  the  fault  plane — not  as  implying  real  displace- 
ment. This  restricted  usage  involves  no  wide  departure  from  that 
of  the  past,  but  it  emphasizes  that  which  has  too  often  been  over- 
looked, i.  e.,  that  the  terms  have  reference  essentially  to  displace- 
ments as  they  appear  in  a  two,  dimensional  cross  section. 

APPARENT  AND  REAL  FAULT  DISPLACEMENTS 

Fault  displacements  shown  in  a  two  dimensional  cross  section 
should  be  assumed  to  be  apparent  until  the  actual  displacement  has 
been  proved.  Arrows,  commonly  used  to  indicate  displacements 
upon  cross  sections,  are  often  misleading.  They  show  only  the 

1  Jaggar,  T.  A.,  Jr.,  How  should  faults  be  named  and  classified?    Econ.  Geol., 
Vol.  2,  1907,  pp.  58-62;  Spurr,  J.  E.,  idem,  pp.  182-184,  601-602;  Willis,  Bailey, 
idem,  pp.  295-298;  Gushing,  H.  P.,  idem.  pp.  433-435;  Tolman,  C.  F.,  Jr.,  idem, 
pp.  506-511;  Evans,  John  W.,  idem,  pp.  803-806;  Chamberlin,  T.  C.,  The  Fault 
Problem,  idem,  pp.  585-601,  704-724. 

2  Econ.  Geol.,  Vol.  2,  1907,  p.  585. 


FAULTS 


37 


FIG.  21.  Thrust  fault  relations  produced  by  horizontal  movement. 


FIG.  22.  Normal  fault  relations  produced  by  horizontal  movement. 


38  STRUCTURAL  GEOLOGY 

apparent  displacement  in  the  plane  of  the  cross  section.    They  are 
likely  to  be  assumed  to  show  the  real  displacement. 

Until  the  real  displacement  is  actually  proved,  we  cannot  avoid 
the  consideration  of  any  of  the  possibilities.  The  limiting  cases 
have  been  discussed  on  preceding  pages.  Yet  seldom  indeed  do  we 
keep  a  sufficiently  open  mind  in  this  regard.  To  illustrate,  in 
the  southern  Appalachians  where  there  are  repeated  overthrust 
faults  associated  with  overthrust  folds,  the  structural  facts  are 
commonly  shown  on  vertical  sections  normal  to  the  trend  of  the 
fault  traces  or  of  the  mountain  ranges.  Without  analyzing  the 
possibilities,  we  are  likely  to  assume  that  the  shortening  is  in 
the  plane  of  the  cross  section,  and  may  overlook  the  fact  that  the 
apparent  displacement  shown  in  the  cross  section  may  not  be  the 
real  displacement  and  that  the  same  structural  features  might  have 
been  produced  by  a  couple  of  forces  acting  in  directions  inclined  to 
apparent  shortening,  producing  a  shearing  movement.  If  a  de- 
formed area  of  this  type  be  regarded  as  a  whole  as  a  strain  el- 
lipsoid (see  pp.  16-20)  with  its  longer  dimensions  parallel  to  the 
trend  of  the  range,  there  is  perhaps  less  difficulty  in  realizing  that 
the  deformation  may  have  been  accomplished  either  by  pure  short- 
ening or  by  shearing  (see  pp.  19-20),  or  more  probably  by  some 
combination  of  the  two  limiting  cases. 

Even  the  use  of  the  strain  ellipsoid  may  be  misleading,  if  care 
is  not  taken  to  ascertain  whether  the  longest  principal  axis  is 
vertical  or  horizontal  or  inclined.  For  instance,  any  ellipse  super- 
posed upon  the  Appalachian  area  with  its  longer  axis  parallel  to 
the  range  is  a  cross  section  of  an  ellipsoid.  It  must  not  be  as- 
sumed that  the  longer  axis  of  this  ellipse  is  really  the  greatest 
principal  axis  of  the  ellipsoid.  In  other  words,  the  extension  may 
have  been  greater  upward  than  along  the  trend  of  the  range.  If 
it  is  true  that  fractures  develop  along  the  planes  of  no  distortion  in 
a  strain  ellipsoid  and  that  the  thrust  faults  of  the  Appalachians  are 
contro  led  by  this  law,  then  it  follows  that  the  longest  axis  of  the 
strain  ellipsoid  must  have  been  essentially  vertical.  This  is  a 
natural  expectation,  for  there  are  reasons  to  believe  that  the 
relief  1  ias  been  easier  upward  than  laterally. 

With  alternative  hypotheses  open,  how  may  the  actual  dis- 
placement be  ^ascertained?  It  is  frequently  impossible  to  do 
this  but  there  are  certain  ways  in  which  the  actual  displacement 


DETERMINATION   OF  REAL   DISPLACEMENTS    39 

has  in  some  localities  been  worked  out.  These  ways  are  as 
follows: 

Striations  may  mark  the  direction  of  displacement.  In  some 
cases  in  repeated  movements  later  striations  have  destroyed 
earlier  ones,  perhaps  formed  in  different  directions. 

The  matching  of  the  ends  of  broken  dikes  often  makes  it  possible 
to  determine  the  actual  displacement  of  faults.  This  method  has 
been  used  very  effectively  by  the  geologists  of  the  Geological 
Survey  of  Scotland  in  determining  both  the  direction  and  amount 
of  the  displacement  of  some  of  the  large  overthrust  faults  of  the 
northwest  Highlands  of  Scotland.  Careful  petrographic  discrimi- 
nation of  these  dikes  and  their  uniformity  and  trend  has  aided 
greatly  in  tracing  the  dikes  individually  and  in  sets. 

The  matching  of  displaced  ore-bearing  veins  has  often  indicated 
the  actual  displacement  of  faults.  Probably  in  few  other  cases 
have  the  displacements  of  faults  in  three  dimensions  been  con- 
sidered so  carefully  as  they  have  in  many  mining  camps.  The 
student  is  referred  to  Weed's  monograph  on  the  Butte  district1 
and  Emmons'  and  Carrey's  bulletin  on  the  Bullfrog  district2  for 
quantitative  studies  of  actual  fault  displacements. 

NORMAL  FAULTS 

Under  this  heading  are  considered  faults  in  which  the  apparent 
displacement  is  downward  on  the  overhanging  side.  In  some  cases 
the  apparent  displacement  is  known  to  be  the  real  displacement — 
in  other  cases  it  is  not. 

Ordinarily  a  normal  or  gravity  fault  is  regarded  as  the  expres- 
sion of  tension,  and  a  reverse  or  thrust  fault  as  evidence  of  com- 
pression; but,  as  noted  under  the  preceding  headings,  the  elonga- 
tion and  shortening  expressed  by  the  terms  normal  faulting  and 
reverse  faulting  have  reference  to  the  relations  of  the  beds  ex- 
pressed in  a  plane  normal  to  the  fault  plane.  When  considered  in 
three  dimensions,  normal  faulting  may  not  show  any  extension  of 
the  mass  as  a  whole,  and  reverse  faulting  may  not  show  any 
shortening  of  the  mass  as  a  whole.  Hinge  faulting  about  an  axis 
may  produce  on  one  side  normal  fault  relations  and  on  the  other 

1  Weed,  W.  H.,  Geology  and  ore  deposits  of  the  Butte  district,  Montana:  Prof. 
Paper  U.  S.  Geol.  Survey  No.  74,  1912. 

2Ransome,  F.  L.,  Emmons,  W.  H.,  and  Garrey,  G.  H.,  Geology  and  ore  deposits 
of  the  Bullfrog  district,  Nevada:  Bull.  407,  U.  S.  Geol.  Survey,  1910. 


40 


STRUCTURAL   GEOLOGY 


reverse  fault  relations  although  there  may  have  been  no  differential 
movements  of  the  centers  of  mass  of  the  two  parts  of  the  faulted 
body.  Horizontal  movement  alone  may  result  in  apparent  normal 
and  reverse  faults.  (See  Figs.  19,  20,  21,  22,  24,  25.) 

Also  faults  which  may  prove  to  be  tension  phenomena  may 
be  merely  subsidiary  expressions  of  a  major  compressive  thrust. 
Sometimes  it  is  necessary  to  know  only  the  actual  displacement  of 


FIG.  23.  To  illustrate  hinge  faulting.  This  would  appear  as  a  normal  or  gravity 
fault  on  a  plane  normal  to  the  fault  plane  passing  through  the  ends  of  the  blocks 
nearest  the  reader  and  as  a  thrust  or  reverse  fault  in  a  plane  passing  through  the 
ends  of  the  blocks  farthest  from  the  reader. 

a  minor  portion  of  the  faulted  mass,  as  for  instance,  in  a  mine, 
regardless  of  any  relation  to  major  deformation.  Ordinarily, 
however,  it  is  desirable  to  relate  the  minor  faulting  to  major 
deformation,  and  this  is  a  much  more  difficult  problem.  An  ex- 
treme case  cited  by  Chamberlin1  is  that  of  a  fault  passing  through 
the  slope  of  a  hill  and  displacing  talus  blocks.  Knowledge  of  the 
relative  displacement  of  the  blocks  in  the  talus  slope  may  give 

1  Op.  cit.,  p.  589. 


NORMAL   FAULTS 


41 


little  clue  to  the  major  and  controlling  displacement.  In  almost 
any  complexly  faulted  area  the  local  displacements  may  be  varied 
and  yet  the  major  and  controlling  displacement  be  a  comparatively 
simple  phenomenon.  A  great  thrust  fault  resulting  in  uplift  may 
be  accompanied  by  a  considerable  variety  of  local  displacements 
which  would  be  interpreted  locally  as  both  thrust  and  tension 
faults.  These  are  subsidiary  and  local  phenomena  due  to  relaxa- 


FIG.  24.  Block  dislocated  by  heave  fault,  showing  apparent  reverse  faulting  of 
bed  BB.    After  Ransome. 

tional  movement,  to  the  concurrent  action  of  gravity,  and  to 
other  causes. 

Whether  a  given  fault  is  really  tensional  or  compressional  when 
considered  in  three  dimensions,  whether  it  is  subsidiary  to  a  major 
fault  of  different  displacement,  has  been  satisfactorily  determined 
in  comparatively  few  instances.  While  the  terms  tension  and 
compression  are  freely  applied  to  faults,  this  is  really  done  on  the 
unreliable  assumption  that  the  apparent  displacement  in  a  vertical 
plane  represents  the  actual  displacement. 

Means  of  identifying  tension  joints  discussed  on  pp.  22-23  may 
be  used  also  for  determining  local  tension  faults. 


42 


STRUCTURAL   GEOLOGY 


Normal  Faults  Associated  with  Igneous  Rocks:  —  Faults  are  likely 
to  be  numerous  within  and  adjacent  to  areas  of  igneous  activity. 
They  are  especially  numerous  in  surface  volcanics.  Such  faults  are 
more  or  less  irregular  and  discontinuous,  and  offset  along  cross 
faults  and  joints,  breaking  the  rocks  into  heterogeneous  polygonal 
blocks.  Displacements  are  both  horizontal  and  vertical.  Normal 


B 


FIG.  25.  Bloek  dislocated   by  movement  between  heave  and  upthrust,  showing 
apparent  normal  faulting.    After  Ransome. 

faults  predominate.  Hinge  faults  are  not  uncommon.  These 
faults  are  well  illustrated  on  many  maps  of  western  mining  districts 
prepared  by  the  U.  S.  Geological  Survey,  notably  those  of  the  Ton- 
opah,1  Goldfield,2  Bullfrog,3  and  Clifton  4  districts. 

1Spurr,  J.  E.,  Geology  of  the  Tonopah  Mining  District,  Nevada:  Prof.  Paper 
No.  42,  U.  S.  Geol.  Survey,  1905. 

2  Ransome.  F.  L.,  Geology  and  ore  deposits  of  Goldfield,  Nevada:  Prof.  Paper 
No.  66,  U.  S.  Geol.  Survey,  1909. 

3  Ransome,  F.  L.,  Emmons,  W.  H.,  and  Garrey,  G.  H.,  Geology  and  ore  deposits 
of  the  Bullfrog  district,  Nevada:  Bull.  407,  U.  S.  Geol.  Survey,  1910. 

4  Lindgren,  Waldemar,  Copper  deposits  of  the  Clifton-Morenci  district,  Arizona: 
Prof.  Paper  No.  43,  U.  S.  Geol.  Survey,  1905. 


NORMAL  FAULTS  43 

It  has  long  been  suspected  that  there  is  some  genetic  connection 
between  faulting  and  igneous  activity.  Spurr  expressed  this 
specifically  as  follows: 1  "It  is  plain  that  the  faulting  was  the 
result  of  adjustments  of  the  crust  to  suit  violent  migrations  of 
volcanic  rock;  that  it  originated  with  the  swelling  up  of  the  crust 
and  its  forcible  thrusting  up  and  aside  to  make  way  for  the  numer- 
ous columns  of  escaping  lava;  and  that  after  the  cessation  of  the 
eruptions  it  was  continued  by  the  irregular  sinking  of  the  crust 
into  the  unsolid  depths  from  which  the  lavas  had  been  ejected.  It 
can  readily  be  seen  that  all  sorts  of  pressure  (from  below  upward, 
lateral,  and  downward,  by  virtue  of  gravity)  must  have  been 
concerned  in  such  movements,  and  that  the  first  faults  were  due 
rather  to  upward  and  lateral  irregular  thrusts,  while  the  later 
ones  (in  many  cases  along  the  same  planes  as  the  first)  were  due 
to  gravity.  So  reversed  and  normal  faults  are  equally  natural,  and 
both  occur  frequently." 

"The  writer  at  first  looked  upon  the  faulting  at  Tonopah  as 
exceptional  and  local,  and  not  to  be  connected  with  ordinary 
faulting  in  the  Great  Basin,  but  there  now  appears  no  reason  for 
doubting  that  the  phenomena  within  this  small,  carefully  studied 
area  are  typical  of  the  unstudied  similar  volcanic  region  beyond 
the  limits  of  the  map." 

In  discussing  joints,  attention  has  been  called  to  the  common 
development  of  joints  and  partings  during  the  cooling  of  igneous 
rocks,  including  peripheral,  radial,  concentric,  basaltic,  and  ir- 
regular partings.  Faulting  may  follow  any  of  these  surfaces  of 
weakness. 

Normal  Faults  in  Unfolded  Sediments: — Normal  faults  may  be 
locally  developed  in  nearly  flat-lying  sediments.  Here  the  cause  of 
tension  may  be  shrinkage  and  settling  due  to  drying  and  recrystall- 
ization.  Often  no  other  causes  are  discernible,  but  it  is  not  possible 
to  exclude  hypotheses  of  regional  or  deep-seated  tension  related  to 
major  earth  movements. 

Association  of  Normal  Faults  with  Folds: — The  reconstruction  of 
an  area  with  abundant  normal  faults  may  develop  a  fold  or  dome  of 
low  slope,  suggesting  that  the  normal  faults  result  from  the  action 
of  gravity  upon  a  mass  elevated  by  folding  but  inadequately  sup- 
ported. Normal  faults  associated  with  overthrust  folds,  to  be 

1  Op.  cit.,  p.  80. 


44  STRUCTURAL   GEOLOGY 

seen,  for  instance,  in  the  southern  Appalachians,  seem  to  be  the 
natural  consequence  of  settling  following  disturbance  of  equilib- 
rium by  thrust,  in  other  words,  of  relaxation  so  commonly  fol- 
lowing compression. 

The  attempt  has  been  made  also  to  correlate  tension  faults 
existing  over  a  great  area  with  the  collapse  of  a  very  gentle  arch. 
For  instance,  the  great  normal  faults  in  the  Great  Basin  area  are 
referred  to  the  collapse  of  an  arch  originally  extending  from  the 
Wasatch  on  the  east  to  the  Sierra  Nevadas  on  the  west.1  Where 
broad,  gentle  arches  are  thrown  up  through  compression  or  through 
changes  in  support  below,  the  inherent  weakness  of  the  rocks  may 
cause  them  to  break  almost  from  the  start  and  allow  certain  blocks 
to  settle  within  the  arch.  Chamberlin2  has  called  attention  to  the 
inherent  weakness  of  rocks  and  their  inability  to  support  them- 
selves in  large  masses.  For  instance,  a  dome  80  miles  in  diameter, 
of  any  thickness,  with  the  curvature  of  the  earth,  will  bear  only  1Us 
of  its  own  weight.  It  is  therefore  apparent  that  when  any  great 
earth  movement  is  initiated,  tending  to  arch  any  part  of  the  earth's 
surface,  unless  this  arch  is  thoroughly  and  evenly  supported  by 
great  masses  below,  it  will  be  unable  to  sustain  itself  by  its  own 
strength  alone;  and  one  would  expect  a  settling  of  blocks,  giving 
the  tension  or  normal  type  of  faulting  and  jointing,  with  conse- 
quent extension  of  surface. 

No  such  relation  as  that  discussed  in  the  above  paragraph  has 
been  proved  on  any  large  scale.  The  existence  of  such  a  primary 
arch  or  tendency  for  arching  is  inferred  as  a  possibility  from  the 
existence  of  supposed  tension  faulting. 

Vertical  and  Steeply-Dipping  Normal  Faults  and  Joints  in  Inter- 
secting Systems: — While  thrust  faults  with  low  dips  are  frequently 
related  to  overthrust  folds  and  have  been  usually  ascribed  to 
horizontal  compression,  in  many  cases  such  pressures  have  also 
been  held  responsible  by  some  geologists  for  intersecting  systems 
of  vertical  or  steeply-dipping  faults  and  joints.3  Becker4  called 
attention  to  the  fact  that  it  is  mechanically  possible  for  inter- 

1  Gilbert,  G.  K.,  Report  on  the  geology  of  portions  of  Nevada,  Utah,  California, 
and  Arizona,  examined  in  the  years  1871  and  1872:  U.  S.  Geog.  Surveys  W.  100th 
Mer.,  Vol.  3,  1875,  pp.  54-56. 

2  Chamberlin,  T.  C.,  and  Salisbury,  R.  D.,  Geology,  Vol.  1,  1904,  p.  555. 

3  Hobbs,  W.  H.,  The  Newark  system  of  Pomperaug  Valley,  Conn.:  21st  Ann. 
Rept.,  U.  S.  G.  S.,  pt.  3,  1901,  pp.  7-162. 

4  Becker,  Geo.  F.:  Bull.  Geol.  Soc.  Amer.,  Vol.  4,  1893,  p.  50. 


NORMAL   FAULTS   IN   INTERSECTING  SYSTEMS    45 

sec  ting  vertical  or  steeply-dipping  faults  and  joints  to  develop 
under  horizontal  compressive  stresses  only  when  lateral  relief  were 
easier  than  upward  relief,  and  that  such  conditions  prevail  over 
certain  areas.  (See  pp.  28-29)  Many  other  geologists  ap- 
parently have  not  analyzed  the  subject  and  have  overlooked  this 
qualification  of  Becker's,  having  assumed  that  the  fact  of  inter- 
section in  sets  implies  compressive  stresses,  without  considering 
alternative  hypotheses. 

The  possibilities  of  lateral  relief  rather  than  upward  relief  are 
difficult  to  determine  in  the  field,  and  ordinarily  it  is  not  possible  to 
be  sufficiently  certain  about  these  to  draw  inferences  from  them  as 
to  the  origin  of  the  fault  by  tension  or  compression.  Irregularities 
of  surface,  like  valleys,  may  permit  easy  lateral  expansion  in  inter- 
vening ridges,  thus  allowing  the  formation  of  vertical  intersecting 
faults  or  joints  by  compressive  stresses.  However,  in  these  cases  it 
is  altogether  likely  that,  for  each  vertical  fracture,  the  comple- 
mentary fracture  (see  pp.  27-28)  may  be  a  horizontal  shearing 
fracture  rather  than  another  vertical  one,  and  that  the  existence  of 
intersecting  vertical  fractures  may  be  due  to  tension  acting  simul- 
taneously or  successively  from  two  or  more  horizontal  directions. 
The  intersecting  vertical  joints  then  have  purely  fortuitous  rela- 
tions. They  do  not  intersect  at  definite  angles  determined  by  the 
shearing  stresses. 

On  the  assumption  that  vertical  or  steeply  dipping  joints  and 
faults  are  the  result  of  tension,  there  have  been  two  explanations  to 
account  for  their  existence  in  intersecting  sets  or  systems.  One 
explanation  is  that  they  were  developed  by  torsion  (see  page  15). 
The  other  explanation  is  that  they  are  the  results  of  successive 
earthquake  shocks  from  different  directions,  in  which  case  they 
form  under  the  tensional  component  of  the  wave,  normal  to  the 
direction  of  propagation  of  the  earthquake  wave  (see  pp.  67-68). 
Still  other  explanations  are  possible.  Joints  and  faults  formed  by 
the  cooling  of  an  igneous  mass,  or  the  settling  and  drying  of  a 
sediment  may  be  in  more  or  less  regular  sets.  Relaxational  settling 
after  a  period  of  compressive  faulting  or  folding  may  develop  nor- 
mal or  steeply  dipping  joints  and  faults  in  intersecting  sets.  The 
systems  in  these  cases  are  not  likely  to  be  uniform  and  yet  for  small 
areas  may  have  a  considerable  regularity  of  arrangement. 

Another  explanation  is  that  one  of  the  vertical  sets  may  be 


46 


STRUCTURAL   GEOLOGY 


tensional  and  the  intersecting  set  may  be  compressional,  in  the 
manner  determined  experimentally  (see  p.  16). 

REVERSE  OR  THRUST  FAULTS 

Sections  in  the  southern  Appalachian  folios  show  thrust  faults 
associated  with  overthrust  folds.  The  fault  planes  may  be  in- 
ferred to  have  the  relations  to  stress  indicated  on  pp.  20-21,  in 
rotational  or  shearing  compressive  strains.  The  inference  is 
usually  made  that  there  is  an  overthrust,  and  therefore  shortening, 
of  so  many  feet.  This  is  true  in  the  plane  of  the  section.  It  tells 
us  nothing  of  the  movements  inclined  to  the  plane  of  the  section, 


FIG.  26.  Overthrust  faulting  localized  by  tension  fracture  "break  thrust."  After 
Willis.  1.  Shows  break  in  the  massive  limestone  bed  which  determines  the 
plane  of  the  break  thrust  along  which  the  displacement  shown  in  2  takes  place. 


which  may  have  been  fully  as  great.  The  association  of  "  thrust" 
faults  with  overthrust  folds  usually  indicates  compression,  but  not 
so  much  compression  as  a  two-dimensional  cross  section  might 
indicate.  Consideration  of  many  cross  sections  is  the  same  in  effect 
as  considering  the  fault  in  three  dimensions,  and  leads  to  closer 
estimates  of  actual  shortening. 

An  examination  of  the  United  States  Geological  Survey  folios 
brings  out  this  interesting  fact  that  in  the  southern  Appalachians 
83%  of  the  thrust  faults,  as  indicated  on  the  cross  sections,  are 
definitely  related  to  overthrust  folds.  Willis1  classifies  them  as 
(1)  break  thrusts  where  the  thrust  fault  plane  follows  a  previously 
formed  tension  fracture  on  the  crest  of  the  anticline;  (2)  shear  or 
stretch  thrusts,  when  the  break  follows  the  sheared  and  stretched 

1  Willis,  Bailey,  Mechanics  of  Appalachian  Structure:  13th  Ann.  Rept.  U.  S. 
G.  S.,  pt.  2,  1893,  pp.  222-223. 


47 


FIG.  27.  Illustrating  thrust  fault  developed  by  stretching  and  by  erosion.  After 
Willis.  1.  Stretch  thrust  developed  from  an  overturned  fold  by  stretching 
of  the  middle  limb;  2.  Erosion  profile  and  section  of  a  simple  anticline;  3.  Ero- 
sion thrust  developed  from  the  condition  shown  in  2  by  compression  from  the 
plateau  side,  accompanied  by  continued  erosion. 


48  STRUCTURAL   GEOLOGY 

underlimb  of  an  overturned  fold ;  and  (3)  erosion  thrusts  where  the 
competent  layer  carrying  the  thrust  is  first  weakened  by  erosion 
at  or  near  the  crest  of  the  anticline.  (Figs.  26  and  27.) 

Distributive  Thrust  Faults: — Not  uncommonly  faulting  takes 
place  along  several  parallel  closely-spaced  planes — in  a  fault  zone 
rather  than  in  a  single  fault  plane.  The  relations  to  stress  are  the 
same  as  in  thrust  faults.  The  total  displacement  may  be  large  and 
yet  the  relative  displacement  along  single  planes  may  be  slight. 
This  faulting  has  been  called  distributive  faulting.  It  is  similar  to 


FIG.  28.  Fault-slip  cleavage  in  gneiss  from  southern  Appalachians.  The  gneiss  has 
been  closely  crenulated  and  the  minute  folds  may  be  observed  to  pass  into 
minute  faults  which  now  represent  planes  of  fracture  cleavage.  The  faults 
may  have  been  cemented  or  may  have  been  welded  by  actual  pressure.  Parallel 
to  the  faults  there  has  also  been  developed  a  parallel  arrangement  of  the 
mineral  particles,  perhaps  due  in  part  to  the  slipping  along  the  fault  planes, 
and  it  is  exceedingly  difficult  to  distinguish  between  the  fracture  cleavage  and 
the  flow  cleavage. 

the  "Schuppen  "  structure  of  the  Germans.  It  is  well  illustrated 
in  the  southern  Appalachians.  Fig.  28  shows  some  of  these  dis- 
tributive faults  associated  with  minute  overthrust  folds.  Distrib- 
utive faults  may  be  on  a  minute  scale,  a  dozen  of  them  being  seen 
in  a  single  hand  specimen,  or  on  an  indefinitely  larger  scale.  Inspec- 
tion of  the  Roan  Mountain  folio  of  the  U.  S.  Geological  Survey  of 
eastern  Tennessee  and  western  North  Carolina  shows  a  remarkable 


DISTRIBUTIVE   THRUST   FAULTS  49 

series  of  parallel  faults  which  on  a  large  scale  can  be  regarded  as 
distributive  faults. 


FIG.  29.  Major  fault  plane  or  fault  sole.    After  Gadell. 

In  the  northwestern  Highlands  of  Scotland  is  a  similar  phenome- 
non, there  called  imbricate  or  schuppen  structure.  The  fault  planes 
are  in  shearing  planes  formed  by  compression  in  a  rotational  strain 
(see  pp.  16-21).  The  beds  are  minutely  sliced  and  piled  one 
on  top  of  the  other.  As  the  deformation  continues  these  beds 
may  ride  forward  as  a  group  over  a  major  fault  plane  at  the  bot- 
tom, sometimes  called  the  "sole."  The  reports  and  maps  of  the 
British  Geological  Survey1  on  the  Scottish  highlands  afford  an 
unrivaled  opportunity  for  the  study  of  faults  of  this  type.  Experi- 
mental reproductions  of  these  faults  by  Cadell 2  throw  light  on  the 
process.  (Fig.  29).  Some  of  his  conclusions  are  quoted : 

1.  Horizontal  pressure  applied  at  one  point  is  not  propagated 
far  forward  into  a  mass  of  strata. 

2.  The  compressed  mass  tends  to  find  relief  along  a  series  of 
gently-inclined  thrust-planes,  which  dip  towards  the  side  from 
which  pressure  is  exerted. 

3.  After  a  certain  amount  of  heaping-up  along  a  series  of  minor 
thrust-planes,  the  heaped-up  mass  tends  to  rise  and  ride  forward 
bodily  along  major  thrust-planes. 

4.  Thrust-planes  and  reversed  faults  are  not  necessarily  devel- 
oped from  split  overfolds,  but  often  originate  at  once  on  application 
of  horizontal  pressure. 

5.  A  thrust-plane  below  may  pass  into  an  anticline  above,  and 
never  reach  the  surface. 

1  Peach,  B.  N.f  Home,  John,  Gunn,  W.,  Clough,  C.  T.,  and  Hinxman,  L.  W., 
The  geological  structure  of  the  northwest  highlands  of  Scotland,  with  petrological 
chapters  and  notes  by  J.  J.  H.  Teall,  edited  by  Sir  Archibald  Geikie:  Mem.  Geol. 
Survey  of  Great  Britain,  1907. 

2  Op.  cit.,  pp.  473-476. 


50  STRUCTURAL   GEOLOGY 

6.  A  major  thrust-plane  above  may,  and  probably  always  does, 
originate  in  a  fold  below. 

7.  A  thrust-plane  may  branch  into  smaller  thrust-planes,  or 
pass  into  an  overfold  along  the  strike. 

8.  The   front  portion  of  a  mass  of  rock  being  pushed  along 
a  thrust-plane  tends  to  bow  forward  and  roll  under  the  back 
portion. 

9.  The  more  rigid  the  rock,  the  better  will  the  phenomena  of 
thrusting  be  exhibited. 

10.  Fan-structure  may  be  produced  by  the  continued  compres- 
sion of  a  single  anticline. 

11.  Thrust-planes  have  a  strong  tendency  to  originate  at  the 
sides  of  the  fan. 

FAULTS  WITH  HORIZONTAL  DISPLACEMENTS 

The  faulting  in  which  the  California  earthquake  originated 
followed  a  vertical  plane  along  which  the  rocks  were  horizontally 
displaced.  This  is  one  of  the  few  cases  of  definitely  proved  hori- 
zontal displacement.1  Illustrations  on  a  much  smaller  scale  may 
be  found  in  the  faulting  of  the  igneous  rocks  of  many  western 
mining  districts,  cited  on  pages  42-43.  Striations  on  fault  surfaces 
not  uncommonly  show  that  there  has  been  some  degree  of  horizon- 
tal displacement,  even  though  the  major  displacement  is  vertical. 
More  attention  is  now  given  than  formerly  to  possibilities  of 
horizontal  displacement,  with  the  result  that  more  information  in 
regard  to  this  type  of  movement  is  becoming  available.  As  yet, 
however,  good  illustrations  are  few. 

HINGE  OR  PIVOTAL  FAULTS 

A  common  type  of  faulting  is  displacement  about  an  axis  normal 
to  the  fault  plane,  one  part  of  the  block  going  up  and  the  other 
part  going  down.  (See  Fig.  23.)  If  the  fault  plane  is  inclined, 
pivotal  faulting  may  give  an  apparent  normal  fault  on  one  side  of 
the  axis  and  an  apparent  reverse  fault  on  the  other  side.  Faults 
of  this  kind  are  numerous  in  the  areas  of  surface  volcanics  in  the 
West.  (See  map  of  the  Iron  Springs  District,  Utah,  Bull.  338, 
U.  S.  G.  S.) 

1  Gilbert,  G.  K.,  Bull.  324,  U.  S.  Geol.  Survey,  1907,  p.  4. 


FOLDED   FAULTS  51 

CURVED  AND   FOLDED  FAULTS 

Curved  fault  and  joint  surfaces,  especially  joint  surfaces,  may 
be  formed  by  spalling  of  surfaces,  caused  by  insolation,  and  other 
processes.  Fractures  related  to  the  cooling  of  igneous  rocks  may  be 
curved.  Curved  fractures  are  found  in  other  relations  where  it 
is  not  easy  to  analyze  causes,  though  there  is  no  reason  to  doubt 
that  they  are  governed  by  principles  already  described. 

After  fracture  planes  are  formed,  they  may  be  faulted  or  folded. 
Folded  thrust  fault  planes  are  described  and  figured  by  Keith  in 
the  Roan  Mountain  folio  of  the  southern  Appalachians  l  (Figs.  30, 
31  and  32),  and  by  Richards  and  Mansfield2  in  the  Bannock  over- 
thrust  in  southeastern  Idaho.  When  previously  fractured  rocks 
undergo  conditions  of  flowage,  the  fractures  are  obliterated. 

Folded  fault  planes  should  not  be  confused  with  the  curving 
of  fault  lines  on  the  erosion  surface,  due  to  irregularities  of  topog- 
raphy. 

FAULTS  PASSING  INTO  FOLDS  OR  INTO  SCHISTOSE  ZONES 

A  fault  may  pass  into  a  fold  along  the  strike,  down  the  dip,  or 
even  up  the  dip.  The  intimate  relation  of  thrust  faults  and  over- 
thrust  folds  has  already  been  cited  for  the  southern  Appalachians. 
Cadell  in  his  experimental  work  illustrating  the  faults  of  the 
Scottish  Highlands  showed  that  the  displacement  by  faulting 
below  might  take  place  above  by  folding.3  The  Kaibab  fault  of 
the  high  plateaus  of  Utah,  a  normal  fault,  grades  along  the  strike 
into  a  monocline. 

Below  the  surface  fractures  die  out,  at  depths  varying  with  the 
strength  of  rocks,  when  the  zone  of  flowage  for  these  given  rocks  is 
reached.  Displacement  may  be  accomplished  by  rock  fracture 
above  and  by  rock  flowage  below.  If  a  cube  of  soft  clay  be  com- 
pressed from  one  side,  held  stationary  at  the  ends,  and  with  room 
for  escape  upward,  a  thrust  fault  will  be  developed  on  its  upper 
side  dipping  toward  the  thrust.  Lower  in  the  cube  this  thrust 

iQeol.  Atlas  U.  S.,  Roan  Mountain  folio,  No.  151,  U.  S.  Geol.  Survey,  1907. 

2  Richards,  R.  W.,  and  Mansfield,  G.  R.,  The  Bannock  overthrust;  a  major  fault 
in  southeastern  Idaho  and  northeastern  Utah:  Jour.  Geol.,  Vol.  20,  1912,  pp.  681- 
709. 

3  Cadell,  H.  M.,  Geological  Structure  of  northwest  Highlands  of  Scotland:  Mem. 
Geol.  Survey  of  Great  Britain,  1907,  pp.  473-476. 


52 


STRUCTURAL   GEOLOGY 


FIG.  30.  Map  of  the  faults  in  the  Roan  Mountain  and  adjacent  quadrangles, 
Tennessee  and  North  Carolina,  showing  the  relation  of  the  minor  faults 
(lighter  lines)  to  the  earlier  major  overthrust  (heavy  line).  Curved  fault  traces 
result  from  folding  and  unequal  erosion.  After  Keith. 


jC>V.\VA«N? 


FIG.  31.  Theoretical  section  across  Buffalo  Mountain  and  Limestone  Cove,  Ten- 
nessee. After  Keith.  Shows  the  character  of  the  deformation  and  the  rela- 
tion of  the  younger  faults  to  the  older  overthrust.  Major  overthrust,  heavy 
continuous  line;  minor  faults,  broken  heavy  line;  Oa,  Athens  shale  and  over- 
lying beds;  COk,  Knox  dolomite;  Cl,  Cambrian  limestones  and  shales;  Cq, 
Cambrian  quartzites  and  slates;  ^llg,  Archean  granite  and  gneiss. 


FIG.  32.  Theoretical  section  showing  supposed  relations  of  beds  in  Fig.  31  after  the 
major  faulting  but  before  the  later  folding  and  faulting.    After  Keith. 


CORRELATION   OF  FAULTS  53 

fault  will  grade  into  a  deformation  which  approximates  rock 
flowage. 

Chamberlin1  has  suggested  that  certain  great  thrust  planes, 
such,  for  instance,  as  the  one  described  by  Willis  in  the  Front 
Range  of  the  Rockies  in  Montana2  may  be  the  equivalent  of 
deformation  by  flowage  down  the  dip  of  the  fault  plane.  Van  Hise 
and  Chamberlin  have  both  regarded  as  probable  the  slipping  of  an 
outer  brittle  and  competent  zone  of  fracture  over  a  lower  zone  of 
flowage  by  tangential  shearing  in  the  upper  part  of  the  zone  of 
flowage.  Chamberlin  would  regard  thrust  faults  as  merely  the 
surface  manifestations  of  this  deep-seated  shearing. 

CORRELATION  OF  FAULTS 

The  complexity  of  fault  phenomena  makes  it  difficult  to  dis- 
cover true  causes  or  displacements.  For  the  same  reason,  it  is 
hardly  legitimate  to  infer  extensions  or  correlation  of  faults  be- 
tween separated  areas.  In  only  a  few  districts  are  the  fault  direc- 
tions sufficiently  uniform  to  warrant  their  correlation  with  faults 
of  substantially  the  same  directions  in  other  districts.  Especially 
is  the  extension  and  correlation  of  faults  unwarranted  in  regions 
of  igneous  rocks  where,  as  shown  by  the  various  maps  of  western 
mining  districts  (such,  for  instance,  as  the  Tonopah,  Clifton, 
Globe,  and  Bisbee)  faults  run  in  nearly  all  directions,  intersect  at 
all  angles,  change  their  directions,  are  cut  off  suddenly,  and  in 
fact,  show  all  the  irregularities  to  be  expected  from  interior  strains 
of  intrusion  and  cooling.  One  is  scarcely  warranted  in  one  of  these 
camps  in  extending  a  fault  on  the  map  ten  feet  beyond  where 
definite  evidence  of  it  is  seen,  for  it  may  suddenly  end  or  change  its 
direction  entirely.  Scarcely  less  irregular  are  the  joints  caused  by 
drying  and  settling  of  sediments.  When  one  considers  the  hetero- 
geneity of  rocks  taken  on  a  large  scale,  it  is  to  be  expected  that  even 
though  the  stresses  are  applied  in  a  uniform  direction  over  a  large 
area,  these  stresses  will  be  carried  and  resolved  in  such  directions 
and  intensities  as  to  develop  fractures  with  great  variety  of  atti- 
tudes. Hence  the  difficulty  of  correlating  faults  over  wide  areas  or 

1  Chamberlin,  T.  C.,  The  fault  problem:  Econ.  Geol.,  Vol.  2,  1907,  pp.  585-601; 
704-724. 

2  Willis,    Bailey,    Stratigraphy   and    structure,    Lewis    and    Livingston   ranges, 
Montana:  Bull.  Geol.  Soc.  Amer.,  Vol.  13,  1902,  pp.  331-336. 


54  STRUCTURAL   GEOLOGY 

between  heterogeneous  systems  of  rocks  can  scarcely  be  over- 
estimated. 

Moreover,  after  rocks  have  been  fractured  they  may  be  de- 
formed by  folding,  in  which  case  the  fault  and  joint  planes  may  be 
so  distorted  that  they  will  appear  on  the  surface  as  curved  lines. 
The  folded  thrust  fault  planes  in  the  southern  Appalachians 
illustrate  the  remarkable  complexity  which  may  be  developed  in  a 
joint  or  fault  plane.  Topographic  irregularities  cause  a  fault  plane 
with  low  dip  to  appear  curved  on  the  surface.  The  surface  distri- 
bution of  such  folded  faults  has  little  similarity  to  the  idealized 
sets  of  straight  line  intersecting  faults  often  presented  as  typical 
of  fault  conditions. 

RELATIVE  NUMBER  OF  NORMAL  AND  REVERSE  FAULTS 

The  prevailing  impression  is  that  normal  faults  are  more  com- 
mon than  reverse  or  thrust  faults,  as  indicated  by  the  use  of  the 
term  "normal."  Chamberlin  and  Salisbury  estimate  that  prob- 
ably 90%  of  the  known  faults  are  normal.1  A  compilation  made 
from  all  the  faults  indicated  in  the  cross  sections  of  U.  S.  Geological 
Survey  folios  fails  to  show  such  large  dominance  of  normal  faults. 
Whatever  the  true  relative  abundance,  it  should  be  kept  in  mind 
that  this  comparison  only  covers  cases  of  apparent  displacement 
in  a  vertical  plane.  It  is  likely  that  faults  with  nearly  horizontal 
displacement  are  much  more  abundant  than  has  been  supposed. 

A  subject  for  inquiry  is  suggested  in  the  relative  abundance 
of  normal  and  thrust  faults  in  rocks  which  have  been  deformed 
only  at  the  surface,  as  compared  with  rocks  which  have  been  de- 
formed deep  below  the  surface  and  subsequently  exposed  by  ero- 
sion. Casual  inspection  of  the  available  data,  particularly  the 
frequent  association  of  thrust  faults  with  phenomena  of  the  zone 
of  rock  flowage,  suggests  that  thrust  faults  are  more  common  in 
rocks  which  have  been  deformed  deep  below  the  surface,  while 
normal  faults  seem  to  be  characteristic  of  surface  deformation. 
Normal  faults  may  imply  extension  of  area  which  is  possibly  only 
at  the  surface.  Of  course  there  can  be  no  clean-cut  discrimination 
of  two  zones.  When  thrusts  are  exposed  by  erosion  they  majr 
have  superposed  on  them  normal  faults  characteristic  of  surface 
conditions. 

1  Chamberlin,  T.  C.,  and  Salisbury,  R.  D.,  Geology,  Vol.  1,  p.  498. 


LENGTHENING   AND   SHORTENING   OF   FAULTS    55 

RELATIVE  SHORTENING  AND  ELONGATION  OF  THE  EARTH'S 
CRUST  BY  FAULTING 

Detailed  studies  of  actual  fault  displacements  are  so  few  and  far 
between  that  little  can  be  said  as  to  the  actual  elongation  or  short- 
ening of  large  parts  of  the  faulted  earth's  crust.  Until  this  is  done, 
it  is  perhaps  premature  to  consider  general  questions  like  the 
shortening  or  elongation  of  the  earth's  crust  in  a  faulted  area. 
Attempts  have  been  made  which  suggest  some  of  the  following 
tentative  and  rather  vague  considerations. 

The  displacements  in  normal  faults  may  be  assumed  to  be 
dominantly  radial  with  regard  to  the  globe,  and  as  the  dip  of 
the  fault  plane  is  seldom  exactly  vertical,  the  downward  move- 
ment requires  extension  of  the  horizontal  surface.  Compression 
faults  may  be  supposed  in  general  to  represent  tangential  shorten- 
ing, with,  subsidiary  vertical  displacement. 

In  view  of  the  difficulties,  already  cited,  of  determining  locally 
whether  a  fault  represents  tension  or  compression  in  three  dimen- 
sions, it  is  obviously  impossible  yet  to  answer  the  question  for 
large  areas  as  to  the  quantitative  effect  of  faulting  on  the  extension 
or  shortening  of  the  earth's  surface.  For  a  given  area  tension 
faults  at  the  surface  may  be  much  more  numerous  than  thrust 
faults,  yet  the  lengthening  of  the  surface  represented  by  the  tension 
faults  may  be  of  less  amount  than  the  shortening  of  the  surface  by  a 
thrust  fault.  The  dip  of  a  thrust  fault  plane  is  usually  low,  that  of 
a  normal  fault  plane,  high.  An  average  from  the  United  States 
Geological  Survey  folios  gives  a  dip  of  36°  for  reverse  fault  planes 
and  78°  for  normal  fault  planes.  A  displacement  of  a  foot  on  the 
thrust  fault  plane  means  nearly  a  foot  of  horizontal  shortening;  a 
displacement  of  a  foot  on  the  normal  fault  plane  means  but  a  few 
inches  of  horizontal  lengthening.  A  single  thrust  plane  of  low  dip, 
then,  may  accomplish  a  horizontal  shortening  which  would  require 
for  compensation  a  large  number  of  normal  faults. 

If  the  crust  as  a  whole  has  been  shortened  by  mountain  folds,  it 
might  appear  that  thrust  faults  are  probably  the  dominant  struc- 
ture, and  that  all  tension  faults  are  ultimately  subsidiary  phenom- 
ena. 

Geologic  history  seems  to  point  to  alternations  of  great  compres- 
sive  and  relaxational  movements.  During  a  period  of  mountain- 
making,  compressive  stresses  develop,  resulting  in  tangential 


56  STRUCTURAL   GEOLOGY 

deformation  in  a  comparatively  short  space  of  time,  with  sub- 
sidiary radial  deformation  in  areas  of  uplift.  During  the  succeed- 
ing period  of  quiescence  it  may  be  supposed  that  the  action  of 
gravity  on  uplifted  areas  may  develop  normal  faults  which  par- 
tially compensate  for  the  earlier  shortening. 

The  extension  of  areas  caused  by  normal  faults  due  to  the  cooling 
of  igneous  rocks  or  the  drying  and  settling  of  sediments  is  com- 
mensurate with  the  shrinkage  of  these  rocks  during  these  processes ; 
such  faults  cause  no  real  extension  of  the  earth's  surface. 

There  have  been  some  attempts  to  calculate  the  lengthening  or 
shortening  of  an  area  on  the  assumption  that  the  displacements 
shown  in  cross  sections  are  the  real  displacements,  without  taking 
into  account  the  probability  of  displacement  in  the  third  dimen- 
sion. One  of  the  few  attempts  to  consider  the  problems  in  three 
dimensions  is  that  of  Emmons  and  Garrey  who  have  estimated  the 
actual  extension  by  faulting  of  the  Bullfrog  district  of  Nevada.1 
They  show  that  the  apparent  extensions  in  individual  cross  sec- 
tions are  greater  than  the  real  extensions  because  there  has  been 
much  movement  in  directions  inclined  to  the  plane  of  the  cross 
section  shown  by  striations  on  fault  surfaces.  From  somewhat 
careful  quantitative  study  they  conclude  that  the  apparent  exten- 
sion should  be  reduced  by  at  least  one-third  to  approximate  the 
real  extension  of  the  area. 

EVIDENCE  OF  FAULTING 

The  existence  of  faults  may  be  determined  by : 

1 .  Fault  scarps,  where  the  faults  are  recent,  and  erosion  has  not 
had  time  to  reduce  them.    Excellent  examples  are  the  Hurricane 
fault  scarp  of  the  Wasatch  front  and  the  scarps  so  conspicuous  in 
the  Basin  Ranges. 

2.  Linear  features  in  the  topography  may  be  caused  by  faulting 
which  brings  into  juxtaposition  rocks  of  differing  resistance  to 
erosion. 

3.  Area!  distribution  of  rocks  or  of  erosion  forms  follows  certain 
general  laws,  the  variation  from  which  requires  the  consideration 
of  faulting  as  a  disturbing  factor.     Illustrations  may  be  found  in 
any  faulted  district. 

1  Ransome,  F.  L.,  Emmons,  W.  H.,  and  Garrey,  G.  H.,  Geology  and  ore  deposits 
of  the  Bullfrog  district,  Nevada:  Bull.  407,  U.  S.  Geol.  Survey,  1910,  p.  88. 


EVIDENCE  OF   FAULTING  57 

4.  Erosion  may  develop  drainage  lines  on  fault  planes.     This 
and  the  other  fault  evidences  above  mentioned  are  further  dis- 
cussed on  a  later  page  under  the  heading  "  Surf  ace  expression  of 
Faults." 

5.  Faulting  is  usually  accompanied  by  a  shear  zone  or  the  divi- 
sion of  the  rock  into  slices  parallel  to  the  plane  of  the  fault. 

6.  Faulting  may  be  accompanied  by  brecciation. 

7.  Faulting  may  be  accompanied  by  the  grinding  up  of  the 
rock  into   a   clay-like  mass,   ordinarily   called   " gouge."      Fault 
gouge  is  some  times  really  clay;  more  often,  however,  it  is  the 
ground  up  rock  from  which  the  bases  have  not  been  removed. 

8.  Striations  on  fracture  surfaces  of  course  suggest  faulting. 

9.  Displacements  of  dikes  and  veins  give  some  of  the  most 
easily  recognizable  evidence  of  faulting. 

It  is  seldom  that  any  one  of  the  above  criteria  will  be  entirely 
decisive  in  itself.  Particularly  is  it  true  that  an  apparent  fault 
scarp  should  not  be  accepted  as  conclusive  proof  of  faulting  until 
faulting  has  been  otherwise  substantiated.  Still  less  is  it  true  that 
drainage  lines  can  be  accepted  in  themselves  as  evidence  of  faulting. 
Even  gouge,  breccias,  etc.,  may  be  developed  under  conditions 
other  than  faulting. 

SURFACE  EXPRESSION  OF  FAULTS 

Normal  faults  may  find  expression  at  the  surface  in  escarp- 
ments, fault  traces,  drainage  lines,  or  modified  distribution  of 
the  rocks.  Escarpments  may  appear  wThere  the  displacement  is 
recent  and  erosion  has  not  had  sufficient  opportunity  to  reduce 
the  inequalities  or  where  the  deformation  has  brought  into  juxta- 
position rocks  of  differing  hardness,  thus  permitting  inequality 
of  erosion  on  the  two  sides  of  the  fault  plane.  In  this  case  the 
downthrow  side  of  the  scarp  may  or  may  not  be  the  downthrow  side 
of  the  fault. 

Among  the  best  known  instances  of  faults  still  represented  by 
the  original  escarpments  are  the  Hurricane  fault  separating  the 
Wasatch  Mountains  from  the  Great  Basin,  and  the  faults  of  the 
Great  Basin  ranges,  which  were  originally  classified  by  Gilbert1 

1  Gilbert,  G.  K.,  Report  on  the  geology  of  portions  of  Nevada,  Utah,  California, 
and  Arizona,  examined  in  the  years  1871  and  1872:  U.  S.  Geog.  Surveys  W.  100th 
Mer.,  Vol.  3,  1875,  pp.  17-187. 


58 


STRUCTURAL   GEOLOGY 


as  an  example  of  the  block  type  of  mountains.  A  fault  scarp 
resulting  from  recent  displacement  accompanied  by  earthquakes 
in  Alaska  is  illustrated  in  Fig.  33.  The  criteria  used  by  Gilbert 
for  the  recognition  and  delineation  of  fault  scarps  of  the  Great 
Basin  are  (a)  their  steepness,  (b)  their  association  with  shear  zones 
and  displacement  of  beds,  (c)  displacement  of  plateau  level  as  in 
the  Hurricane  fault  of  Utah,  (d)  the  fact  that  the  scarps  may  not 
converge  toward  the  end  of  the  mountains  as  they  would  if  they 


FIG.  33.  Fault  scarp  developed  during  earthquake  of  1899,  Yakatut  Bay  region  of 
Alaska.    After  Martin. 

were  normal  erosion  scarps,  (e)  the  existence  of  triangular  facets 
across  the  ends  of  ridges  as  though  the  ridge  had  been  sliced  off, 
(f)  the  recent  displacement  of  alluvial  fans,  lake  beds,  and  other 
surface  features,  indicating  that  the  faulting  has  been  going  on  to 
very  recent  times  and  has  not  had  time  to  be  masked  by  erosion. 
Spurr1  questions  these  criteria  for  the  surface  delineation  of  faults, 
or  rather,  the  degree  of  emphasis  to  be  placed  on  them.  He  calls 
attention  to  the  fact  that  erosion  has  been  conspicuously  effective 
in  producing  the  present  topographic  features  of  some  of  the  Great 
Basin  ranges,  that  anticlines  and  synclines  play  an  important 

1  Spurr,  J.  E.,  Origin  and  structure  of  the  Basin  Ranges:  Bull.  Geol.  Soc.  Am., 
Vol.  12,  1901,  pp.  264-266. 


SURFACE   EXPRESSION   OF  FAULTS  59 

part,  and  that  the  recognized  faults  in  these  ranges  are  often 
quite  independent  of  the  topographic  features.  The  student 
may  study  maps  of  these  ranges  to  advantage,  keeping  in  mind  the 
criteria  above  cited.  On  these  and  on  the  Terlingua,  Texas, 
topographic  map,  specific  topographic  features  seem  to  indicate 
recent  faulting. 

In  districts  of  older  deformation,  like  the  southern  Appala- 
chians, erosion  has  had  a  longer  time  to  develop  the  topographic 
features,  with  the  result  that  original  fault  scarps  are  practically 
non-existent.  The  effect  of  faults  on  the  topography  is  due  to  their 
bringing  into  contact  rocks  of  unequal  hardness,  thus  permitting 
differential  erosion.  The  flat  and  curved  attitudes  of  the  fault 
planes  here  also  tend  to  make  them  less  conspicuous  in  the  topog- 
raphy. 

Thrust  faults  are  not  likely  to  produce  steep  vertical  escarp- 
ments, either  before  or  after  erosion.  By  pushing  forward  succes- 
sive slices  of  rock  they  tend  to  cause  linear  features  in  the  topog- 
raphy, and  yet  these  features  are  not  different  from  those  which 
might  have  been  produced  by  folding  and  probably  they  would 
not  be  identified  as  related  to  thrust  faulting  unless  the  thrust 
faulting  had  been  otherwise  proved. 

In  regions  of  vertical  faults,  especially  in  flat-lying  beds  and  non- 
glaciated  areas,  the  lines  of  the  faults  are  very  likely  to  be  marked 
by  drainage  channels  which  have  developed  along  these  planes  of 
weakness.  All  faults  are  not  marked  by  drainage  lines,  nor  do  all 
drainage  lines  mark  faults.  (See  p.  12.) 

One  of  the  most  fully  studied  cases  of  the  surface  expression  of  a 
fault  with  horizontal  displacement  is  that  of  the  fault  which 
caused  the  California  earthquake  of  1906.  "At  the  surface  the 
cracks  had  great  variety  of  expression.  Some  were  barely  percep- 
tible as  partings;  others  gaped  so  widely  that  one  might  look  down 
them  several  yards.  Some  were  mere  pullings  apart ;  others  showed 
small  differential  movements  of  the  nature  of  faulting.  Some  were 
solitary;  others,  especially  those  exhibiting  faulting,  were  in 
groups."  *  Where  the  fault  crossed  a  spur  or  shoulder  of  a  moun- 
tain a  scarp  appears.  Small  basins  or  ponds,  many  having  no 
outlets  and  some  containing  saline  water,  are  frequently  found  at 
the  base  of  small  scarps.  Troughlike  depressions  appear  on  both 

1  Gilbert,  G.  KM  Bull.  324,  U.  S.  G.  S.,  1907,  p.  7. 


60  STRUCTURAL   GEOLOGY 

sides,  also  bounded  by  scarps.  Small  knolls  or  sharp  little  ridges 
are  common  at  the  fault  line  and  these  are  bounded  on  one  side  by 
a  softened  scarp  and  separated  from  the  normal  slope  of  the  valley 
side  by  a  line  of  depression.  Other  effects  of  this  fault  are  slides  of 
earth  or  rock  from  the  hillslopes.  Finally,  there  are  many  con- 
spicuous dislocations  of  the  works  of  men.1 

The  relations  of  valleys  and  particularly  lakes  to  fault  displace- 
ments may  be  studied  to  advantage  in  the  northern  part  of  the 
Santa  Cruz,  California,  folio. 

SUGGESTIONS  FOR  LABORATORY  STUDY  OF  FAULTS 

(See  also  page  31) 

Much  can  be  done  in  the  study  of  faults  on  geologic  maps,  referred  to 
on  the  foregoing  pages,  particularly  in  the  section  on  the  surface  expression 
of  faults.  It  is  suggested  that  these  maps  and  others  named  below  be 
studied  with  a  view  of  answering  specifically  the  following  questions: 

How  are  the  faults  indicated  by  the  topography,  or  the  distribution  of 
the  outcrops?  What  is  the  dip  of  the  fault  plane?  What  is  the  apparent 
displacement?  Is  there  any  way  of  ascertaining  the  real  displacement? 
Considered  in  three  dimensions,  what  has  been  the  deformation  accom- 
plished by  the  faulting?  The  dip  of  a  fault  plane  may  be  determined  in 
some  cases  by  actual  measurement,  in  others  by  the  relation  of  outcrops 
to  topography.  What  are  the  possible  relations  of  the  fault  plane  to 
stresses  producing  it? 

Morristown,  Tennessee,  folio  (No.  27)  U.  S.  Geol.  Survey. 

Roan  Mountain,  Tennessee-North  Carolinia,  folio  (No.  151)  U.  S. 
Geol.  Survey. 

Stratigraphy  and  structure,  Lewis  and  Livingston  Ranges,  Montana, 
by  Bailey  Willis:  Bull.  Geol.  Soc.  Am.,  Vol.  13,  1902,  pp.  305-352. 

Anthracite-Crested  Butte,  Colorado,  folio  (No.  9)  U.  S.  Geol.  Survey. 

Silverton,  Colorado,  folio  (No.  120)  U.  S.  Geol.  Survey. 

Bisbee,  Arizona,  folio  (No.  112)  U.  S.  Geol.  Survey. 

The  Bannock  overthrust;  a  major  fault  in  southeastern  Idaho  and 
northeastern  Utah,  by  R.  W.  Richards  and  G.  R.  Mansfield:  Jour.  Geol., 
Vol.  20,  1912,  pp.  681-709. 

The  interpretation  of  topographic  maps,  by  R.  D.  Salisbury  and  W.  W. 
Atwood:  Prof.  Paper  No.  60,  U.  S.  Geol.  Survey,  1908,  p.  77. 

The  geological  structure  of  the  northwest  Highlands  of  Scotland: 
Memoir  Geol.  Survey,  Great  Britain,  1907,  pp.  463-476. 

Report  on  an  investigation  of  the  geological  structure  of  the  Alps,  by 
Bailey  Willis:  Smithsonian  Misc.  Collections,  Vol.  56,  No.  31,  1912. 

Experiments  on  faulting  must  be  limited  to  the  equipment  available  in 
the  laboratory.  If  there  is  available  equipment  for  compression  tests  of 

1  Lawson,  A.  C.,  Preliminary  report  of  the  State  Earthquake  Investigation  Com- 
mission, 1906,  and  final  report,  1908. 


LABORATORY   STUDY  OF  FAULTS  61 

stone  or  building  materials,  this  can  be  advantageously  used  in  experi- 
ments of  the  kind  referred  to  on  page  16. 

There  are  other  simple  and  inexpensive  devices  for  showing  most  of  the 
facts  discussed  on  the  foregoing  pages.  One  of  these  is  the  wire  screening, 
described  on  pp.  18-20.  It  is  easy  to  devise  apparatus  for  the  deforma- 
tion of  clay  and  small  plaster  of  Paris  blocks,  because  the  stresses  required 
are  very  moderate.  Much  can  be  done  with  these  materials  with  an 
ordinary  vise  or  in  using  clay  with  a  box  with  movable  sides  working  under 
screw  compression. 

Another  simple  device  is  a  rubber  sheet  mounted  on  a  frame  with  ex- 
tension screws,  so  that  it  may  be  stretched  or  contracted  in  any  direction. 
The  sheet  is  coated  with  paraffine  and  stretched.  Tension  fractures 
develop  normal  to  the  stretching.  If  the  rubber  sheet  be  first  extended  in 
one  direction  and  then  coated  with  paraffine  and  allowed  to  contract, 
compression  fractures  develop  in  planes  dipping  toward  or  away  from  the 
compression  or  contraction  and  striking  normal  to  the  direction  of  move- 
ment. Contraction  in  one  direction  is  accompanied  by  expansion  in  a 
direction  normal  to  it,  resulting  in  tension  fractures  normal  to  the  com- 
pression fractures.  This  simple  equipment  also  allows  of  experiments 
involving  warping  and  rotational  stresses. 

The  deformation  on  the  surface  of  an  expanding  or  contracting  rubber 
sheet  is  perhaps  more  nearly  like  rock  deformation  in  nature  than  the 
deformation  produced  by  applying  external  pressure  through  the  sides  of 
a  rectangular  box,  for  so  far  as  the  conditions  can  be  inferred  in  nature, 
the  stresses  are  ordinarily  not  applied  externally  against  definite  faces  of 
the  deformed  mass,  but  are  distributed  throughout  considerable  rock 
masses. 

FRACTURE  CLEAVAGE  AND  FISSILITY 

Fracture  cleavage  may  be  defined  as  a  structure  inherent  in  a 
rock  mass  whereby  under  stress  it  breaks  along  closely  spaced 
parallel  incipient  joints.  It  differs  from  flow  cleavage  in  features 
noted  below.  The  term  fissility  has  been  used  by  Van  Hise1  for 
the  actual  parallel  partings;  but  he  uses  it  also  to  include  capac- 
ity to  part  along  such  parallel  planes.  In  the  latter  usage  it  is 
practically  synonymous  with  fracture  cleavage.  It  may  be  desir- 
able to  retain  the  term  fissility  as  strictly  defined  by  Van  Hise 
for  the  actual  partings,  as  distinguished  from  fracture  cleavage, 
which  applies  to  the  capacity  to  part.  Other  terms  more  or  less 
synonymous  with  fracture  cleavage  are  close-joints  cleavage,  "aus- 
weichungs"  cleavage,  fault-slip  cleavage,  rift,  etc.  (Figs.  34-37). 

Fracture  cleavage  is  a  fracture  phenomenon  and  is  developed 

1  Van  Hise,  C.  R.,  Principles  of  North  American  pre-Cambrian  geology:  16th 
Ann.  Kept.  U.  S.  G.  S.,  pt.  1,  1896,  p.  033. 


62  STRUCTURAL   GEOLOGY 

under  the  general  stress-strain  relations  already  discussed  for 
joints  and  faults.  In  some  cases  the  surfaces  of  weakness  are 
clearly  cemented  joint  surfaces.  In  other  cases  there  is  no  evidence 
that  there  has  ever  been  actual  parting  followed  by  cementation; 
the  surfaces  seem  to  be  incipient  fracture  surfaces  along  which  the 
rock  is  still  coherent,  like  cracks  in  a  plate  which  has  not  yet  fallen 
apart.  Arrangement  of  the  mineral  particles  with  their  longer 
axes  in  the  plane  of  fracture  cleavage  is  not  a  necessary  condition, 
though  this  arrangement  is  often  secondarily  developed  by  rubbing 
between  the  parts.  Fracture  cleavage  may  be  partly  the  result  of 


FIG.  34.  Fracture  cleavage  developing  polygonal  blocks  in  slate  previously  possess- 
ing flow  cleavage. 

minute  relative  displacements  along  incipient  fracture  planes  by 
minor  monoclinal  folding  or  faulting  of  the  distributive  type  men- 
tioned in  another  place.  (See  p.  48  and  Fig.  35.) 

Fracture  cleavage  planes  are  more  widely  spaced  than  "flow 
cleavage"  planes  (see  p.  76)  and  are  characteristically  in  two 
or  more  intersecting  sets,  allowing  the  rock  to  break  into  various 
polygonal  forms.  In  some  rocks  one  set  is  so  dominant  and  so 
closely  spaced  as  to  give  a  structure  very  closely  simulating  flow 
cleavage;  indeed,  there  are  many  rocks  in  which  the  structure 
cannot  be  satisfactorily  designated  either  as  fracture  or  flow 
cleavage,  but  is  in  reality  some  combination  of  the  two. 

Fracture  cleavage,  as  a  phenomenon  of  the  zone  of  rock  frac- 


FRACTURE   CLEAVAGE  63 

ture,  may  be  superposed  upon  rocks  which  had  before  been  in  the 
zone  of  rock  flowage.  The  previous  existence  of  a  good  rock  cleav- 
age developed  in  the  zone  of  flow  favors  the  development,  in  the 


FIG.  35.  Photomicrograph  of  slate  with  false  or  fracture  cleavage  from  Black  Hills 
of  South  Dakota.  The  longer  diameters  of  the  particles,  mainly  mica,  quartz, 
and  feldspar,  lie,  for  the  most  part,  in  a  plane  intersecting  the  plane  of  the  page 
and  parallel  to  its  longer  sides,  but  in  well-separated  planes  at  right  angles  to 
this  plane  the  longer  diameters  of  the  particles  have  been  deflected  into  minute 
monoclinal  folds  represented  by  the  darker  cross  lines.  In  these  cross  planes 
also  porphyritic  biotites  have  developed  with  their  longer  diameters  parallel. 
The  rock  has  two  cleavages,  one  conditioned  by  the  prevailing  dimensional 
arrangement  of  the  minute  particles  and  the  other  conditioned  by  the  planes  of 
weakness  along  the  axes  of  the  minute  monoclinal  folds  crossing  the  prevailing 
cleavage.  The  first  cleavage  is  flow  cleavage  developed  in  normal  fashion 
during  rock  flowage,  and  the  second  is  of  the  nature  of  fracture  cleavage 
developed  later  along  separated  shearing  planes  in  the  zone  of  fracture  or  in 
the  zone  of  combined  fracture  and  flowage.  The  rock  cleaves  into  parallele- 
piped blocks. 

zone  of  fracture,  of  closely-spaced  parallel  planes  of  parting, 
yielding  fracture  cleavage  or  fissility. 

On  the  other  hand,  if  a  rock  with  fracture  cleavage  comes  into 
the  zone  of  rock  flowage,  the  structure  is  obliterated. 

A  common  example  of  the  development  of  fracture  cleavage 


64 


STRUCTURAL   GEOLOGY 


or  fissility  is  found  where  a  soft  bed  is  deformed  by  fracture  be- 
tween two  stronger  beds,  as  for  instance,  the  Baraboo  quartzite. 
Here  curved  fissures  are  formed  by  compression  (see  Figs.  9,  10, 
and  11),  and  these  are  crossed  by  tension  cracks.  The  mechanics 
of  this  problem  are  discussed  on  pp.  16-21. 


FIG.  36.     Fracture  cleavage  crossing  flow  cleavage.     After  Dale. 

BRECCIAS  AND  AUTOCLASTICS 

When  rocks  are  broken  into  irregular  angular  fragments  they 
are  called  " breccias/'  "friction  breccias,"  or  "  autoclastics" 
("  self  -broken"  rocks).  They  may  be  cemented  by  infiltration. 
Such  rocks  may  be  difficult  to  discriminate  from  conglomerates 
or  "elastics"  formed  by  the  ordinary  processes  of  erosion.  Some 
of  these  differences  are  as  follows:  (a)  The  fragments  in  the  auto- 
clastic  rock  are  usually  more  angular  than  those  of  the  con- 
glomerate, but  to  this  there  are  exceptions,  (b)  They  are  likely  to 
be  more  homogeneous  in  character;  ordinarily  they  are  of  one  kind 
of  rock.  Clastic  rocks  may  have  several  kinds  of  fragments  coming 
from  different  sources.  However,  many  elastics  are  made  up 
dominantly  of  one  kind  of  fragment;  hence  this  criterion  is  also 


BRECCIAS   AND   AUTOCLASTICS 


65 


inconclusive,  (c)  The  cement  of  an  autoclastic  rock  is  likely  to  be 
vein  material,  while  that  of  a  clastic  is  usually  fine-grained  frag- 
mental  material.  This  is  one  of  the  safest  criteria,  (d)  An  auto- 
clastic  rock  may  be  developed  in  zones  crossing  the  bedding.  This 


FIG.  37.  Fracture  cleavage,  jointing  and  flow  cleavage  developed  in  graywacke 
and  slate,  Alaska.  After  Gilbert  (photograph  by  U.  S.  Geol.  Survey.)  Use 
principle  of  strain  ellipsoid  (See  pp.  18-21)  to  ascertain  direction  of  relative 
displacement  and  theoretic  position  of  fracture  planes  and  flow  planes.  (See 
also  Figs.  9  and  10). 

occurrence  sometimes  gives  a  clue  as  to  its  origin.  No  one  of  the 
above  distinctions  is  decisive.  Collectively  they  may  be  so,  but 
not  in  all  cases. 


66  STRUCTURAL   GEOLOGY 

In  so  far  as  autoclastics  and  conglomerates  have  been  rendered 
schistose,  the  difficulty  of  discrimination  is  increased.  An  Archean 
porphyry  of  the  Vermilion  district  of  Minnesota  is  sheared  into 
rhombs,  which  have  been  flattened  by  flowage,  and  brought 
out  by  weathering  as  elongated  lenses,  like  pebbles.  This  pseudo- 
conglomerate  can  be  discriminated  only  with  the  greatest  difficulty 
from  the  overlying  true  conglomerate  forming  the  base  of  the 
Algonkian  which  is  made  up  dominantly  of  pebbles  of  porphyry, 
likewise  elongated  by  flowage. 

Volcanic  tuffs  and  breccias  often  simulate  autoclastic  rocks  in 
many  respects.  Tuffs  result  from  fracture,  but  differ  from  auto- 
clastics in  that  the  fracture  is  caused  by  volcanic  explosion  rather 
than  by  mechanical  stresses  acting  directly  through  the  earth. 
Volcanic  "flow  breccias"  produced  by  the  cooling,  hardening,  and 
breaking  up  of  the  lava  surface  while  still  flowing,  likewise  resemble 
autoclastics.  Means  of  identification  of  tuffs  and  volcanic  breccias 
are  often  found  (1)  in  the  homogeneity  of  their  fragments,  (2)  in  the 
possession  of  volcanic  textures  such  as  amygdules  peripherally 
arranged,  (3)  in  the  cementation  of  their  fragments  by  volcanic 
dust  or  rock,  and  (4)  in  the  angularity  of  the  fragments.  None  of 
these  criteria  are  decisive  in  discriminating  tuffs  from  elastics  or 
autoclastics.  Tuffs  formed  under  water  are  distinguished  only 
with  very  great  difficulty  from  water-deposited  elastics  resulting 
from  erosion.  The  fragmental  rocks  associated  with  the  immense 
basaltic  flows  of  Ontario  and  the  Lake  Superior  region  well  illus- 
trate the  difficulties  of  this  discrimination.1 

Autoclastics  may  be  confused  with  volcanic  rocks  having  amygda- 
loidal  fillings  or  porphyritic  textures  especially  when  the  volcanics 
are  schistose,  and  also  with  concretionary  structures  of  limestone. 

It  may  seem  that  the  above  discussion  of  the  character  of  auto- 
clastics emphasizes  too  strongly  the  difficulties  of  their  discrimina- 
tion from  conglomerates,  tuffs,  and  other  rocks.  It  has  been  the 
writer's  experience,  however,  that  these  difficulties  have  been  a 
much  too  common  source  of  mistake  and  confusion,  especially  in 
schistose  areas.  Hasty  judgments  based  on  the  superficial  aspect 
of  the  rock  lead  to  unreliable  results.  Criteria  should  be  applied  in 
detail  and  the  conclusion  verified  by  all  possible  geologic  evidence. 

1  Van  Hise,  C.  R.,  and  Leith,  C.  K.,  Geology  of  the  Lake  Superior  region:  Mon. 
52,  U.  S.  G.  S.,  1911,  pp.  118-143. 


BRECCIAS   AND   AUTOCLASTICS  67 

A  geologist  who  has  been  working  in  an  area  where  there  is  clear 
evidence  of  autoclastics  may  carry  preconceived  notions  of  the 
abundance  of  that  kind  of  rock  into  another  area,  and  fail  to 
recognize  there  the  existence  of  a  conglomerate  of  great  structural 
significance.  Another,  who  has  dealt  principally  with  conglomer- 
ates and  seen  little  of  autoclastics,  may  assume  a  rock  to  be  a 
conglomerate  without  sufficiently  considering  the  possibilities  of 
its  being  autoclastic.  Owing  to  the  bias  of  preconceived  opinions 
various  interpretations  of  the  origin  of  the  fragmental  rocks  at  the 
base  of  the  Huronian  series  in  the  "  Original  Huronian"  district 
north  of  Lake  Huron  for  a  long  time  caused  controversy  and 
delayed  a  true  understanding  of  the  geology  of  this  important 
area.  The  base  of  the  Algonkian  in  northern  Minnesota  is  made  up 
of  fragments  of  the  underlying  Archean  basalts  and  porphyries, 
which  still  retain  their  angular  form  and  evidently  have  been  but 
little  worn  and  transported.  Lying  unconformably  beneath  them, 
and  associated  with  the  basalts  and  porphyries,  are  various  auto- 
clastic  and  tuffaceous  forms  so  similar  in  characteristics  to  the 
basal  conglomerate  that  even  with  the  application  of  the  most 
careful  criteria,  it  is  frequently  difficult  or  impossible  to  tell  whether 
a  given  exposure  of  rock  should  be  mapped  as  Algonkian  or 
Archean.  In  the  progress  of  the  mapping  the  interpretation  of 
the  geology  has  changed  from  time  to  time  according  as  these 
rocks  came  to  be  better  known. 

Autoclastic  and  clastic  rocks  should  be  studied  with  an  open 
mind,  and  with  an  appreciation  of  the  varied  possibilities  of  origin. 
When,  after  all  criteria  have  been  used,  there  is  still  uncertainty, 
this  should  be  clearly  indicated,  in  order  to  keep  the  subject  open 
for  further  investigation. 

EARTHQUAKES 
EARTHQUAKES  AS  CAUSE  AND  EFFECT  OF  ROCK  FRACTURE 

Earthquakes  are  not  in  themselves  rock  structures,  but  are  the 
accompaniments  and  results  of  rock  fracturing. 

Earthquakes  may  be  also  the  cause  of  fracturing.  Crosby  1  has 
argued  that  earthquakes  are  one  of  the  important  causes  of  joint- 
ing, the  joints  developing  normal  to  the  direction  of  propagation 

Crosby,  W.  O.,  Am.  Geol.,  Vol.  12,  1893,  pp.  368-375. 


68  STRUCTURAL   GEOLOGY 

of  the  wave  by  the  tensional  component.  Intersecting  sets  there- 
fore would  require  successive  earthquake  waves  from  different  di- 
rections. He  further  has  shown  experimentally  that  where  rocks 
are  already  under  strain  the  earthquake  wave  may  bring  the 
stresses  beyond  the  breaking  point  simultaneously  in  many  parts 
of  the  rock  mass,  the  joints  occurring  in  planes  determined  by  the 
initial  strain,  or,  in.  a  sudden  and  violent  shock,  in  planes  deter- 
mined by  the  direction  of  the  earthquake  wave.  So  far  as  the 
writer  knows,  there  are  comparatively  few  cases  of  joint  systems 
which  can  be  definitely  proved  to  be  related  to  earthquakes, 
notwithstanding  the  inherent  probability  that  earthquakes  ac- 
complish such  results,  and  notwithstanding  known  associations  of 
earthquakes  with  a  plane  or  zone  of  faulting  (see  p.  58.)  Independ- 
ent of  any  real  bearing  that  earthquakes  may  have  on  jointing,  it 
is  to  be  remembered  that  the  actual  stress-strain  relations  at  the 
point  of  rupture  must  fall  within  the  range  of  the  limiting  cases  of 
tension  and  compression  already  described.  So  far  as  the  earth- 
quake merely  accentuates  the  strain  already  present,  it  is  obvious 
that  the  strain  may  be  either  the  result  of  tension  or  compression. 
So  far  as  the  earthquake  wave  itself  develops  the  strain,  it  seems 
likely  also  that  both  tensional  and  compressive  joints  might  be 
expected,  although  actual  proof  of  one  or  the  other  is  difficult  to 
cite.  Earthquake  waves  may  vibrate  parallel  or  normal  to  the 
direction  of  transmission.  Those  vibrating  parallel  to  the  direction 
of  transmission  may  cause  both  compression  and  tension.  The 
question  difficult  to  answer  is,  which  of  these  waves,  or  which 
component  of  these  waves,  first  surpasses  the  breaking  strength  of 
the  rock?  In  general  rocks  may  be  expected  to  yield  to  tension 
first. 

The  effects  of  earthquakes  on  building  and  other  structures  need 
not  be  detailed  from  our  point  of  view,  because  they  do  not  con- 
stitute a  part  of  the  geological  record  under  consideration.  They 
are  of  interest  from  a  geological  standpoint  as  showing  the  direction 
of  transmission  and  vibration  of  earthquake  waves,  (see  page  74) 
the  displacements  along  faults,  etc.  The  maximum  shaking  and 
destructive  effects  of  earthquakes  appear  to  be  in  loosely  con- 
solidated rocks,  gravels,  and  soils  which  are  saturated  with  water. 
The  reason  for  this  is  not  entirely  clear.  It  has  been  suggested 
that  the  water  affords  opportunity  for  the  materials  to  move 


EARTHQUAKES  69 

easily,  and  by  filling  all  the  pore  spaces  that  it  aids  in  the  trans- 
mission of  the  shock. 

Observations  taken  in  sounding  and  on  the  breaks  in  cables 
following  earthquakes  have  shown  that  large  segments  of  the 
bottom  of  the  ocean  have  dropped  hundreds  of  feet  as  a  result  or 
cause  of  such  shocks.  Ordinarily,  the  accompanying  continental 
changes  have  been  of  smaller  magnitude  and  usually  uplifts 
rather  than  depressions.  Continental  changes,  while  considerable, 
are,  as  listed  by  Milne,  commonly  measured  by  units  of  a  few  feet  or 
a  few  tens  of  feet. 

Within  our  zone  of  observation  earthquakes  are  clearly  re- 
lated to  rock  fracturing,  but  it  is  not  certain  that  they  may  not 
also  have  relation  to  sudden  deformation  by  rock  flowage  at  points 
below  our  observation.  When  it  is  remembered  how  intimate  is  the 
association  of  fracturing  and  rock  flowage,  how  the  two  processes 
seem  in  some  places  to  go  on  side  by  side  under  the  same  stresses, 
it  becomes  obviously  difficult  to  exclude  rock  flowage  from  con- 
sideration in  connection  with  earthquakes. 

Rock  flowage  as  a  result  of  earthquake  shock  is  even  more  prob- 
able. If  local  stresses  are  almost  of  the  necessary  magnitude  to 
produce  rock  flowage,  it  is  conceivable  that  the  earthquake  shock 
may  carry  these  stresses  past  the  resistance  of  the  rock  and  require 
rock  flowage.  Also,  it  may  be  supposed  that  rock  flowage  already 
started  may  be  accelerated  by  earthquake  shocks. 


KINDS  OF  FRACTURING  ACCOMPANYING  EARTHQUAKES 

It  is  not  easy  to  correlate  earthquake  shocks  with  particular 
kinds  of  fractures.  It  is  the  natural  assumption  that  faults  with 
great  displacements  cause  great  earthquakes,  yet  so  far  as  histori- 
cal records  go,  great  earthquakes  have  sometimes  been  associated 
with  apparently  slight  breaks.  There  is  nothing  yet  to  show  that 
earthquakes  are  associated  alone  with  compressive  or  with  tension 
fractures.  So  far  as  field  observations  inform  us,  earthquakes  are 
most  frequently  related  to  vertical  fissures,  but  certainly  one  must 
suppose  that  great  thrust  faults  initiate  earthquakes.  The  fact 
that  so  many  earthquakes  can  not  be  definitely  connected  with 
fractures  at  the  surface  may  indicate  their  relation  to  thrust  faults 
forming  beneath  the  surface.  Milne  regards  the  minor  earth- 


70  STRUCTURAL   GEOLOGY 

shakers  or  "  microseisms "  as  the  result  of  minor  settlings  near 
the  surface,  along  previously  formed  vertical  fissures.1 

EARTHQUAKES  AND  OSCILLATIONS  OF  GLACIERS 

Glacial  oscillations  have  been  shown  recently  to  be  related  to 
earthquake  shocks,2  making  it  possible  to  interpret  certain  re- 
markable advances  of  glaciers  otherwise  inexplicable. 

EARTHQUAKES  AND  VULCANISM 

There  is  a  relationship  between  earthquakes  and  vulcanism,  both 
in  time  and  place.  Vulcanism  has  been  accompanied  in  many 
places  by  earthquakes,  and  vice  versa.  However,  there  are  many 
cases  of  earthquakes  not  associated  with  vulcanism,  and  of  many 
volcanic  outbreaks  not  accompanied  by  earthquakes.  Since 
vulcanism  is  now  generally  regarded  as  involving  mechanical 
disturbances  of  the  crust,  lessening  the  pressure  upon  the  hot  rock, 
and  thereby  allowing  it  to  liquefy,  it  may  be  reasoned  that  earth- 
quakes, by  disturbing  the  equilibrium  of  pressures,  may  be  a  local 
cause  ©f  vulcanism.  Or,  both  may  result  from  larger  earth  move- 
ments. 

EARTHQUAKES  AND   MAGNETIC  DISTURBANCES 

Earthquakes  are  often,  though  not  always,  accompanied  by 
magnetic  disturbances.  There  are  still  differences  of  opinion  as 
to  whether  or  not  these  magnetic  disturbances  are  mere  incidents 
of  the  mechanical  readjustments.  There  is  some  evidence  that  the 
magnetic  disturbance  is  not  entirely  related  to  mechanical  changes, 
suggesting  the  possibility  of  some  further,  and  as  yet  unknown, 
underlying  relationship. 

Another  interesting  relationship,  as  yet  unexplained,  is  observed 
between  some  earthquake  zones  and  regions  of  permanently  steep 
magnetic  gradients  of  the  earth's  magnetic  field. 

EARTHQUAKES  AND  ROCK  DENSITY 

Earthquakes  are  likely  to  be  numerous  in  regions  which  show 
steep  rock  density  gradients,  that  is,  regions  in  which  light  and 
heavy  earth  masses  are  in  close  contiguity. 

1  Milne,  John,  Seismological  observations  and  earth  physics:  Geographical  Jour. 
Vol.  21,  1903,  page  21. 

2  Tarr,  R.  S.,  and  Martin,  Lawrence,  The  earthquakes  at  Yakutat  Bay  in  Septem- 
ber, 1899:  Prof.  Paper  No.  69,  U.  S.  Geol.  Survey,  1912. 


EARTHQUAKES  71 

EARTHQUAKE  ZONES 

The  distribution  of  earthquakes  corresponds  with  zones  of  more 
or  less  intense  deformation  or  vulcanism  or  both.  In  a  very 
general  way  there  are  two  great  earthquake  zones,  the  so-called 
Mediterranean  zone  or  belt  passing  through  the  Himalayas  and 
eastern  China,  from  which  have  started  53%  of  the  recorded  earth- 
quakes; and  the  Pacific  belt  bordering  the  Pacific  basin,  in  which 
have  originated  41%  of  the  recorded  earthquakes.1  More  specif- 
ically, earthquakes  are  likely  to  follow  along  the  margins  of 
continents  or  of  smaller  areas  of  great  relief,  along  mountain  chains, 
especially  of  recent  origin,  along  volcanic  belts,  along  margins  of 
two  areas  differing  considerably  in  density,  as  for  instance  in  the 
zone  of  the  Messina  earthquake,  and  along  areas  where  there  are 
great  irregularities  in  distribution  of  the  earth's  magnetism.  It 
has  been  ascertained  that  earthquakes  have  been  especially  nu- 
merous in  the  geosynclines  of  Mesozoic  rocks.  As  many  of  these 
rocks  have  been  folded  into  mountain  ranges  in  comparative!}'  late 
geological  time,  this  is  only  a  specific  case  of  the  abundance  of 
earthquakes  in  mountains  of  recent  origin. 

INSTRUMENTS  FOR  DETERMINING  AND  MEASURING  EARTH- 
QUAKES 

Seismographs  are  instruments  for  the  detection  and  measuring 
of  earthquakes.  They  are  made  in  -a  variety  of  forms  but  are  all 
essentially  devices  for  determining  more  or  less  independently  the 
three  principal  components  of  a  wave,  that  is,  the  vibrations  in 
three  mutually  perpendicular  planes.  A  pendulum  makes  an  auto- 
matic record,  mechanically  or  photographically,  on  a  sheet  moving 
at  a  uniform  rate  beneath  it.  The  record  is  a  straight  line  until  it  is 
disturbed  by  an  earthquake  wave,  when  the  line  becomes  crenu- 
lated.  The  earthquake  wave  is  expressed  in  the  amplitude  and 
spacing  of  the  crenulations.  It  is  more  correct  to  say  that  the 
wave  is  expressed  partly  on  any  one  record,  for  only  those  com- 
ponents of  the  wave  that  are  normal  to  the  plane  of  the  pendulum 
are  expressed.  In  order  to  get  a  complete  record  there  must  be 
three  seismographs  oriented  in  mutually  perpendicular  directions. 

1  Montessus  de  Ballore,  F.  de,  Les  Tremblements  de  Terre,  Paris,  1906. 


72  STRUCTURAL   GEOLOGY 

EARTHQUAKE   WAVES 

Earthquake  waves  are  supposed  to  be  (1)  compress! ve  or  longi- 
tudinal, vibrating  parallel  to  the  direction  of  transmission  of  the 
shock,  and  (2)  transverse,  vibrating  normal  to  the  direction  of 
transmission  of  the  shock.  Earthquake  waves  reach  a  distant 
point  on  the  earth's  surface  both  by  passing  along  the  chord  and  by 
going  around  the  circumference,  arriving  at  different  times.  The 
circumferential  wave  travels  about  one-third  as  fast  as  the  chord 
wave.  The  chord  wave  travels  through  the  earth's  diameter  in  22 
minutes.  The  chord  waves  are  regarded  as  compressive,  the  cir- 
cumference waves  as  transverse.  Milne  regards  it  as  uncertain 
whether  the  circumferential  wave  is  undulatory  in  vertical  dimen- 
sion, like  a  wave  propelled  in  water,  involving  tilting  of  the  surface, 
or  whether  it  is  distinctly  a  horizontal  shaking.  The  Kingston 
earthquake  sent  out  two  principal  shocks.  The  first  of  these, 
the  one  traveling  along  the  chord,  was  registered  on  a  seismograph 
at  Washington  (almost  due  north),  and  principally  on  the  pendu- 
lum which  vibrated  east  and  west;  the  wave  therefore  was  vibrat- 
ing north  and  south;  it  was  a  compressive  wave.  The  second 
one,  traveling  along  the  circumference  of  the  earth,  was  registered 
principally  on  the  pendulum  vibrating  north  and  south;  the  wave 
was  vibrating  east  and  west;  it  was  a  trans vere  wave.  This  is  the 
kind  of  evidence  upon  which  the  directions  of  earthquake  vibra- 
tions are  determined.  It  is  not  always  regarded  as  conclusive. 

A  third  wave  may  follow  at  an  interval  which  suggests  that  it  has 
gone  around  the  longer  arc  of  the  earth's  circumference.  There  is 
evidence  of  convergence  of  waves  at  antipodal  points,  and  of  their 
resurgence  to  points  of  observation. 

CONDITION  OF  EARTH'S  INTERIOR  AS  INFERRED  FROM  EARTH- 
QUAKE WAVES 

If  earthquake  waves  have  both  circumferential  and  chord  paths, 
their  behavior  points  to  certain  differences  in  the  physical  condition 
of  the  media  they  traverse.  Within  600  miles  of  their  origin,  the 
first  and  second  waves  are  confused.  It  may  be  calculated  that 
neither  of  these  waves  would  get  below  12  miles  from  the  surface 
in  traveling  to  a  point  within  600  miles.  Beyond  600  miles  the  two 
waves  become  sharply  separated  in  time,  suggesting  that  the  chord 


EARTHQUAKES  73 

wave,  which  now  passes  more  than  12  miles  from  the  surface,  is  in  a 
different  kind  of  medium.  It  is  possible  that  in  the  first  instance 
the  waves  were  passing  entirely  through  the  zone  of  rock  fracture ; 
in  the  second  instance  the  deeper  waves  were  passing  partly 
through  the  zone  of  rock  flow.  Milne  also  notes  that  chord  waves 
registered  at  antipodal  stations  and  therefore  traveling  along  a 
path  nearly  through  the  center  of  the  earth,  behave  differently 
from  waves  passing  through  intermediate  depths  or  along  the  sur- 
face. The  former  are  much  dampened  and  confused,  suggesting 
still  a  different  physical  state  at  the  center.  In  general,  however, 
the  deep  or  chord  waves  travel  with  such  a  speed  as  to  indicate  a 
rigidity  nearly  twice  that  of  steel,  and  their  uniform  speed  argues 
for  homogeneity  of  the  medium  traversed.1 

The  evidence  bearing  on  the  nature  and  paths  of  earthquake 
waves  is  complex,  and  agreement  has  not  been  reached  among 
seismologists.  Some  of  the  principal  conclusions  of  seismologists 
on  the  subject  have  been  merely  noted. 

LOCATION  OF  THE  ORIGIN  OF  EARTHQUAKES 

So  far  as  earthquake  shock  results  mainly  from  rock  fracture, 
its  origin  is  in  the  zone  of  rock  fracture  and  hence  probably  not  far 
beneath  the  surface.  Doubtless  there  are  readjustments  in  the 
zone  of  flow  at  the  same  time,  but  these  may  be  subsidiary  as 
causes.  A  shallow  depth  for  the  origin  of  earthquakes  has  been 
found  wherever  it  has  been  possible  to  determine  the  directions  of 
emergence  of  earthquake  waves,  either  from  instrumental  observa- 
tions or  from  the  study  of  the  destructive  results  of  earthquake 
shocks.  Nowhere  have  these  determinations  indicated  a  depth  of 
origin  greater  than  12  miles,  and  usually  less.  The  very  fact  that 
an  earthquake  shock  is  usually  so  well  localized  at  some  spot  on  the 
earth's  surface,  that  there  is  some  one  zone  which  may  be  regarded 
as  the  locus  of  activity,  is  evidence  that  its  origin  is  not  far  below 
the  surface.  These  observations  have  already  been  cited  as 
evidence  that  the  zone  of  rock  fracture  is  not  deep.  Granting  that 
earthquakes  originate  in  the  zone  of  fracture,  it  may  be  argued 
that  their  depth  of  origin  at  the  maximum  is  about  10  or  12  miles, 

1  Milne,  John,  Seismological  observations  and  earth  physics:  Geographical  Jour., 
Vol.  21,  1903,  p.  7. 


74  STRUCTURAL   GEOLOGY 

since  there  is  some  evidence  that  the  zone  of  fracture  does  not 
generally  extend  beyond  that  depth. 

In  the  area  most  affected  by  the  quake,  the  origin  is  located  by 
the  intensity  of  the  shock  and  by  noting  the  direction  of  emergence 
of  the  waves.  The  area  most  affected  is  usually  roughly  oval 
or  elliptical  and  within  it  there  is  usually  a  line  or  spot  at  which 
the  intensity  of  the  shock  is  clearly  at  a  maximum.  It  is  as- 
sumed that  near  at  hand  the  waves  are  both  transverse  and  com- 
pressive,  that  both  shearing  and  tensional  stresses  are  set  up  in  the 
structures  affected,  and  that  the  breaking  strength  is  first  sur- 
passed by  tensional  stresses,  the  dominant  one  of  which  would  be 
normal  to  the  direction  of  transmission  of  the  wave.  Hence 
fracture  planes  in  buildings  are  regarded  as  due  to  tension,  and 
therefore  normal  to  the  path  of  the  wave.  The  plane  of  fracture  is 
best  determined  at  the  corner  of  a  building.  Lines  drawn  normal 
to  these  fracture  planes  in  widely  distributed  areas  may  tend  to 
converge  in  a  point,  or  plane,  which  are  then  regarded  as  the  origin 
of  the  quake.  This  method  is  of  doubtful  value,  because  the 
attitude  of  fractures  is  so  influenced  by  local  conditions,  and  it  is 
difficult  to  prove  that  they  are  tensional. 

The  location  of  the  earthquake  from  more  distant  points  is 
accomplished  mainly  by  noting  the  difference  in  time  of  the  receipt 
of  the  principal  shocks,  chord  and  circumferential.  The  greater 
the  difference  in  time  between  the  receipt  of  the  two  the  greater  the 
distance  from  the  point  of  origin.  At  any  one  point  the  distance, 
not  direction,  is  determined.  It  needs  observations  of  distance 
from  three  points  to  determine,  by  the  intersection  method,  the 
locus  of  origin. 

PREDICTION  OF  EARTHQUAKES 

It  has  not  been  possible  thus  far  to  predict  with  any  considerable 
degree  of  success  the  time  and  place  of  earthquakes.  As  to  place, 
there  is  the  probability  that  earthquakes  will  be  confined  to  certain 
broad  zones  in  which  they  have  commonly  originated  in  the  past. 
Within  an  earthquake  zone  the  records  seem  to  show  that  a  great 
disturbance  at  one  locality  may  mean  that  the  next  disturbance  is 
to  be  looked  for  in  some  other  part  of  the  great  belt.  There  have 
been  too  many  exceptions  to  this,  however,  to  establish  the  rule. 
When  one  notes  the  widespread  distribution  of  faults  over  the 


EARTHQUAKES  75 

earth's  surface,  most  of  them  doubtless  accompanied  in  their 
formation  by  earthquakes,  and  considers  the  possibilities  for 
faulting  in  the  geologic  future,  predictions  as  to  the  localization  of 
earthquakes,  based  on  the  meagre  records  of  historical  time, 
can  not  be  accepted  with  any  great  confidence. 

Attempts  to  establish  a  principle  of  periodicity  for  earthquakes 
have  been  equally  futile.  Gilbert x  calls  attention  to  the  fact  that 
many  attempts  at  working  out  the  periodicity  of  earthquakes  are 
apparently  successful  because  the  great  frequency  of  earthquakes 
of  some  magnitude  furnishes  examples  for  any  time  system  postu- 
lated. 

1  Gilbert,  G.  K.,  Earthquake  forecasts:  Science,  Vol.  29,  1909,  pp.  121-138. 


ROCK  FLOWAGE 

A  rock  is  said  to  have  flowed  when  it  is  deformed  without  con- 
spicuous fracture,  remaining  at  the  end  of  the  deformation  an 
integral  body.  This  interpretation  does  not  exclude  minor  frac- 
tures in  the  constituent  minerals  during  rock  flowage.  Rock 
flowage  produces  hard  and  crystalline  types.  The  process  is 
essentially  a  constructive  and  integrating  one.  As  here  used,  it 
has  no  necessary  relation  to  fusion,  though  it  is  possible  that 
the  high  pressures  involved  may  cause  minerals  to  melt  at  com- 
paratively low  temperatures.1 

One  of  the  conspicuous  results  of  rock  flowage  is  a  slaty  or 
schistose  or  gneissic  structure,  giving  the  rock  a  cleavage.  All 
such  structures  are  described  below  under  the  heading  of  "Flow 
Cleavage."  In  so  far  as  gneissic  structure  shows  banding,  without 
cleavage,  as  it  sometimes  does,  this  is  discussed  under  another 
head  (p.  87).  Some  rocks  flow  without  taking  on  either  a  schistose 
or  slaty  or  gneissic  structure.  These  are  likewise  discussed  under  a 
subsequent  heading.  Fracture  cleavage  or  fissility,  already  dis- 
cussed, is  a  phenomenon  of  rock  fracture  rather  than  of  rock  flow, 

FLOW  CLEAVAGE2 

Flow  cleavage  is  a  capacity  of  some  rocks  to  part  along  parallel 
surfaces,  not  necessarily  planes.  These  surfaces  are  determined  by 
the  parallel  dimensional  arrangement  of  the  mineral  constituents, 
that  is,  by  the  mutual  parallelism  of  the  greatest,  mean,  and  least 
dimensional  axes  of  the  mineral  particles  making  up  the  rock  mass. 
They  may  also  be  determined  by  the  parallelism  of  the  mineral 
cleavages  of  the  constituent  particles. 

A  few  minerals,  such  as  mica,  hornblende,  quartz,  and  feldspar,  in 
various  ratios,  make  up  all  but  a  very  small  percentage  of  schistose 
or  cleavable  rocks.  To  make  the  discussion  concrete,  therefore, 
cleavage  will  be  discussed  principally  in  relation  to  these  four 

1  Johnston,   John,  and  Adams,   L.  H. ,    On  the  effect  of  high'  pressures  on  the 
physical   and    chemical   behavior  of   solids:   Am.   Jour.    Sci.,    vol.   35,    1913,   pp. 
205-253. 

2  For  fuller  discussion  see:  Leith,  C.  K.,  Rock  cleavage:  Bull.  239,  U.  S.  Geol. 
Survey,  1905,  pp.  23-118. 

76 


FLOW   CLEAVAGE  77 

minerals.  The  technical  reader  will  at  once  think  of  qualifications 
and  additions  necessary  where  other  minerals  are  considered,  but 
in  the  writer's  judgment  these  do  not  essentially  affect  conclusions 
based  on  the  study  of  a  few  of  the  principal  schist-forming  minerals. 

One  of  the  peculiar  features  of  a  cleavable  rock  is  the  uniformity 
in  shape  of  the  grains  of  each  of  the  characteristic  minerals,  deter- 
mined by  their  crystal  habit.  The  average  ratio  of  the  greatest 
to  the  mean  dimensions  of  a  mica  plate  is  about  10:1,  of  horn- 
blende 4:1,  and  of  quartz  and  feldspar  1.5:1.  These  ratios  are  the 
same  whether  the  rock  cleavage  is  good  or  poor.  In  other  words, 
the  better  rock  cleavage  does  not  necessarily  mean  a  greater 
drawing  out  or  elongation  of  mineral  particles. 

When  in  the  laboratory  crystals  are  allowed  to  develop  under 
stress,  they  elongate  in  the  plane  of  easiest  relief,  supposedly, 
regardless  of  habit,  but  this  is  not  certain,  because  the  experiments 
have  been  conducted  principally  with  isometric  crystals.1  Also, 
crystals  not  under  conditions  of  growth  have  been  elongated  by 
pressure  alone,  again  more  or  less  regardless  of  habit.  But  not- 
withstanding these  experimental  results,  the  minerals  in  schists 
have  an  elongation  ordinarily  determined  by  habit  alone.  The 
difference  between  a  schist  with  poor  cleavage  and  one  with  good 
cleavage  is  not  so  much  that  the  particles  of  one  have  been  elon- 
gated more  than  the  particles  of  the  other,  but  that  it  has  more  of 
the  kinds  of  particles  which  by  habit  are  elongated. 

There  is,  in  the  schists,  relative  perfection  of  crystal  forms, 
dependent  on  the  character  of  the  minerals,  as  compared  with 
igneous  rocks,  where  shape  of  the  minerals  depends  more  largely 
on  order  of  crystallization.  This  mineral  form  and  arrangement  in 
schists  is  the  "  crystalloblastic  "  structure  of  Milch2  and  Gruben- 
mann.3 

The  parallel  dimensional  arrangement  of  the  mica  and  horn- 
blende, and  sometimes  the  feldspar,  implies  a  parallelism  of  their 
mineral  cleavages,  because  these  minerals  tend  to  occur  with 
definite  crystal  habit  within  the  rock,  and  the  mineral  cleavages 
are  definitely  oriented  with  reference  to  the  dimensional  axes. 

1  Becker,  G.  F,,  and  Day,  A.  L.,  Linear  force  of  growing  crystals:  Proc.  Wash. 
Acad.  Sci.,  Vol.  7,  1905,  pp.  283-288. 

2  Milch,  L.,  Die  heutigeri  ansichten  liber  Wesen  und  Entstehung  der  kristallinen 
Sehiefer:  Geol.  Rundschau,  vol.  1,  1910. 

3  Grubenmann,  IL,  Die  kristallinen  Schiefer,  part  1,  1904,  part  2,  1907. 


78  STRUCTURAL   GEOLOGY 

The  orientation  of  the  dimensional  axes  of  the  particles  therefore 
carries  with  it  an  orientation  of  the  mineral  cleavages.  Mica 
crystals,  for  instance,  lying  dimensionally  parallel  in  a  schist,  have 
their  mineral  cleavages  in  the  plane  of  the  two  greater  dimensional 
axes,  that  is,  in  the  plane  of  rock  cleavage.  Hornblende  crystals 
lie  with  their  long  dimensional  axes  parallel;  the  mean  or  least 
dimensional  axes  of  hornblende  crystals,  being  so  nearly  of  the 
same  length,  may  not  be  parallel.  The  two  cleavages  of  horn- 
blende are  parallel  to  the  major  dimensional  axes,  but  are  inclined 
to  the  minor  dimensional  axes.  Thus  the  hornblende  cleavages 
in  the  schistose  rocks  are  parallel  to  an  axis,  but  not  to  a  plane. 
The  feldspar  habit  does  not  give  such  great  dimensional  differences. 
Most  of  the  feldspars  in  schist  show  only  a  slight  tendency  to  as- 
sume elongated  or  tabular  shapes  due  to  crystal  habit.  Their 
dimensional  arrangement  is  more  or  less  independent  of  crystal- 
lographic  arrangement  and  therefore  there  is  only  a  slight  tendency 
toward  parallelism  of  the  feldspar  cleavages. 

The  dimensional  elongation  of  mica  and  hornblende  parallel  to 
their  cleavage  faces  in  schists  has  been  cited  as  indicating  some  sort 
of  genetic  relationship  between  mineral  elongation  and  mineral 
cleavage.1 

A  schistose  rock  cleaves  either  between  the  mineral  particles, 
following  the  plane  of  their  greatest  and  mean  dimensional  axes, 
or  within  the  mineral  particles  along  their  cleavage  planes.  The 
first  is  known  as  inter-mineral  cleavage,  and  is  a  capacity  to  part 
determined  by  the  dimensional  arrangement  of  mineral  particles; 
the  second  may  be  called  inter-molecular  cleavage,  and  is  related 
to  the  ultimate  molecular  structure  of  the  crystal.  Ordinarily 
when  a  rock  is  cleaved  the  two  surfaces  show  the  glistening  faces 
of  hornblende  or  mica  or  of  other  minerals  of  this  type,  indicating 
that  the  break  has  followed  the  mineral  cleavages.  The  parting 
here  has  obviously  been  easier  than  between  the  mineral  particles. 
In  places  where  mica  and  hornblende  are  not  abundant,  the  cleaved 
surfaces  of  the  rock  show  quartz  and  feldspar,  indicating  that  the 
breaking  has  been  principally  of  the  inter-mineral  type. 

Whatever  the  relative  importance  of  inter-mineral  and  inter- 
molecular  cleavage,  it  should  be  remembered  that  all  mineral 

1  Trueman,  J.  D.,  The  value  of  certain  criteria  for  the  determination  of  the  origin 
of  foliated  crystalline  rocks:  Jour.  Geol.,  Vol.  20,  1912,  pp.  228-258,  300-315. 


FLOW   CLEAVAGE  79 

particles  in  cleavable  rocks  are  dimensionally  arranged,  and 
that  this  dimensional  arrangement  involves  parallelism  of  the 
mineral  cleavages  only  for  part  of  the  minerals.  Therefore  the 
conclusion  is  justified  that  the  dimensional  parallelism  of  mineral 
particles  is  the  controlling  factor  in  rock  cleavage;  that  to  this 
control  is  due  the  mutual  parallelism  of  mineral  cleavages  of  mica 
or  hornblende  cleavages.  Nevertheless  it  may  to  some  extent  be 
true  that  the  cleavages  of  these  minerals  have  some  influence  on 
their  elongation,  and  therefore  on  their  arrangement.  As  the 
dimensions  of  the  minerals  of  schists  are  controlled  by  mineral 
habit,  this  becomes  an  important  factor  in  the  structure  of  schists. 

MANNER  IN  WHICH  THE  PARALLEL  ARRANGEMENT  OF  MIN- 
ERALS  IS   BROUGHT  ABOUT 

The  arrangement  of  the  mineral  constituents  of  a  cleavable 
rock  is  the  result  of  the  differential  pressure,  which  caused  the 
rock  to  flow.  The  conditions  under  which  this  occurred  are  dis- 
cussed on  pp.  4-10.  Briefly,  the  general  conditions  of  rock 
flowage  have  been  found  to  be  great  pressure  from  all  sides,  high 
temperature,  abundance  of  altering  solutions,  susceptibility  of  the 
rock  to  mineral  and  chemical  change  depending  on  its  composition, 
and  slow  deformation;  in  other  words,  rock  flowage,  judging  from 
the  field  and  laboratory  evidence,  is  accomplished  by  means  of 
physical  and  chemical  changes  combined.  These  general  observa- 
tions do  not  indicate  just  in  what  manner  the  parallel  arrangement 
of  mineral  constituents  producing  the  cleavage,  the  most  con- 
spicuous result  of  rock  flowage,  has  been  accomplished. 

Recry stabilization: — A  study  of  cleavable  rocks  shows  that  much 
of  the  hornblende  and  mica,  minerals  which  are  responsible  for 
some  of  the  best  rock  cleavage,  is  of  entirely  new  generation  in  the 
secondary  rock.  A  shale  or  mud  may  have  no  mica;  a  phyllite, 
its  altered  equivalent,  may  have  as  high  as  50%,  by  weight,  of 
mica.  Chemical  analysis  shows  that  this  change  may  occur  in  some 
instances  with  little  addition  or  subtraction  of  materials.  A  correct 
inference  is  that  the  new  minerals  of  the  hornblende  and  mica 
types  have  developed  principally  from  the  recrystallization  of 
substances  already  in  the  rock  mass.  Even  where  there  is  quan- 
titative evidence  that  substances  have  been  introduced  or  ex- 
tracted, the  mass  has  still  been  recrystallized.  Since  hornblende 


80  STRUCTURAL   GEOLOGY 

and  mica  are  the  common  minerals  producing  the  best  rock  cleav- 
age, it  must  be  concluded  that  recrystallization  is  the  important 
process  in  the  development  of  parallelism  of  the  mineral  constit- 
uents. 

Corroborative  evidence  of  the  importance  of  recrystallization  is 
the  general  lack  of  fractures  or  other  strain  effects  in  the  minerals 
of  a  cleavable  rock,  such  as  would  be  expected  if  the  parallelism  had 


FIG.  38.  Photomicrograph  of  micaceous  and  quartzose  schist  showing  recrystal- 
lized  quartz.  From  Hoosac,  Mass.  The  view  illustrates  in  detail  the  relation 
of  recrystallized  quartz  grains  to  recrystallized  mica  flakes.  The  mica  flakes  for 
the  most  part  separate  different  quartz  individuals,  but  they  may  be  seen  to 
bound  two  or  more  individuals  and  to  project  well  into  them.  It  is  not  prob- 
able that  such  a  relation  could  be  brought  about  by  granulation,  slicing,  or 
gliding,  and  it  seems  best  explained  by  recrystallization. 

been  brought  about  entirely  or  largely  by  mechanical  processes. 
It  may  be  inferred,  then,  that  some  constructive  process,  which 
may  be  called  generally  recrystallization,  has  been  at  work. 

Most  of  the  mineral  particles  in  the  cleavable  rocks  are  in- 
dividually larger  than  the  particles  in  the  same  rocks  before 
flowage  had  occurred.  For  instance,  the  gradation  of  a  shale  to  a 
phyllite  means  an  increase  in  the  size  of  the  grains.  Recrystalliza- 
tion is  the  constructive  process  which  has  accomplished  this  result. 


FLOW   CLEAVAGE 


81 


The  cleavable  rock  is  likely  to  show  a  great  uniformity  in  size  and 
shape  of  the  grains  of  the  same  mineral  as  compared  with  the 
non-schistose  rock,  and  again  recrystallization  explains  the  phe- 
nomenon. 


FIG.  39.  Photomicrograph  of  micaceous  schist  from  Hoosac  tunnel.  The  micas, 
which  are  entirely  new  developments  by  recrystallization,  lie  in  flat  plates 
with  their  greater  diameters  roughly  parallel.  Each  individual  exhibits  several 
twinning  lamellae.  It  will  be  noted  that,  while  there  is  apparently  a  bending 
and  irregularity  in  the  mica  plates,  the  individuals  are  for  the  most  part  not 
deformed,  and  the  impression  of  irregularity  is  caused  by  the  individuals 
feathering  out  against  one  another  at  low  angles.  This  sort  of  arrangement  is 
frequently  seen  about  rigid  particles  which  have  acted  as  units  during  deforma- 
tion, indicating  that  the  arrangement  is  due  to  differing  stress  conditions  at 
different  places. 


Much  detailed  microscopical  evidence  might  be  cited,  such  as 
dove-tailing  of  quartz  individuals  in  quartz  bands,  the  feathering 
out  of  mica  plates  against  an  adjacent  mineral  surface,  the  lack  of 
bending  and  breaking  of  hornblende  needles  by  mutual  interference, 
the  segregation  of  minerals  into  bands,  to  show  that  the  parallel- 
ism could  not  have  been  produced  by  mechanical  adjustment 


82 


STRUCTURAL   GEOLOGY 


alone,  but  must  have  been  aided  by  the  chemical  and  mineralogical 
changes  involved  in  recrystallization.    (See  Figs.  38,  39  and  43.) 

Granulation  and  rotation  of  original  particles: — But  recrystalliza- 
tion is  not  the  only  process  instrumental  in  the  production  of  rock 
cleavage.  The  quartz  and  feldspar  in  the  cleavable  rock  may  be 


FIG.  40.  Photomicrograph  of  schistose  quartz-porphyry  showing  sliced  feldspar 
phenocryst  in  planes  inclined  to  the  prevailing  cleavage.  After  Futterer. 
(Fig.  2,  PL  III  of  Ganggranite  von  Grosssachsen  und  die  Quartzporphyre  von 
Thai  in  Thuringer  Wald:  Mitt.  Grossh.  Badischen  geol.  Landesanstalt,  Vol.  2, 
Heidelberg,  1890.) 


largely  original  quartz  and  feldspar;  some  of  the  mica  and  horn- 
blende also  may  be  original.  Parallelism  may  be  partly  due  to 
rotation  from  original  random  positions.  This  process  may  be 
aided  by  granulation  and  slicing  of  the  original  mineral  particles. 
Broken,  unequidimensional  mineral  fragments  are  often  strewn  out 
in  such  a  manner  that  their  longer  dimensions  lie  approximately 
parallel.  Evidence  of  rotation  is  seen  principally  in  the  quartz 


FLOW   CLEAVAGE  83 

and  feldspar,  which  have  not  much  effect  in  producing  rock  cleav- 
age. It  is  concluded,  then,  that  the  rotation  of  original  particles, 
diversely  oriented,  to  a  parallel  position  is  a  minor  factor  quite 
subordinate  to  the  dominant  process  of  recrystallization.  (See 
Figs.  40,  41,  42  and  43.) 

In  the  incipient  stages  of  rock  flowage  the  larger  and  more 
brittle  particles  are  granulated  and  elongated.  At  the  same  time 
recrystallization,  beginning  on  the  finer  particles,  builds  up  new 
minerals.  In  the  intermediate  and  advanced  stages  it  gradually 
dominates  over  granulation  and  ultimately  obliterates  any  evidence 
of  it.  It  may  be  inferred  that  granulation  aids  recrystallization  in 


FIG.  41.  Sliced  feldspars  in  micaceous  and  chloritic  schist  from  southern 
Appalachians. 

that  it  grinds  the  particles  into  small  pieces  and  affords  greater 
surface  upon  which  the  chemical  process  may  act. 

In  experimental  deformation  the  conditions  are  not  favorable 
for  recrystallization,  and  granulation  is  the  important  process. 

Slipping  or  twinning  along  the  cleavage  planes  of  minerals,  called 
"  gliding" — such  as  may  be  observed  in  calcite  and  ice  crystals — 
has  been  cited  as  a  possible  cause  of  the  elongation  and  parallel 
arrangement  of  mineral  particles.  This  has  been  observed  only  in 
minerals  of  the  calcite  type,  which  are  not  important  in  cleavable 
rocks;  and  even  in  the  calcite  of  schistose  rocks  gliding  has  been 
found  to  be  subordinate  to  processes  of  recrystallization  and 
granulation.,  In  experimental  deformation  of  marble  it  seems  to 
play  a  greater  part,  because  conditions  of  recrystallization  are  not 
present. 

There  is  no  evidence  that  the  flattening  of  original  mineral 
particles  to  a  dimensional  parallelism,  without  regard  to  crystallo- 
graphic  arrangement,  has  played  any  important  part  in  the  pro- 
duction of  rock  cleavage;  indeed,  some  of  the  facts  already  cited 


84  STRUCTURAL   GEOLOGY 

constitute  decisive  evidence  to  the  contrary.  Such  is  the  evidence 
that  hornblende  and  mica,  essential  minerals  of  schistose  rocks, 
are  in  many  cases,  and  perhaps  in  most  cases,  entirely  new  develop- 
ments in  the  rock.  Of  the  same  nature  is  the  evidence  derived  from 
the  uniformity  of  dimensional  characteristics  of  the  particles  of  a 
given  mineral  species  and  the  control  of  dimensions  by  crystal 
habit.  The  most  cleavable  rock  is  not  made  up  of  natter  particles 
of  hornblende,  mica,  quartz,  or  feldspar  than  the  less  cleavable 
rock.  But  it  certainly  contains  more  particles  of  hornblende  and 
mica  than  of  quartz  and  feldspar;  consequently  it  has  more  parti- 
cles which  are  flat  or  elongate,  which  give  it  a  better  and  smoother 
cleavage. 

If  this  is  true,  the  development  of  rock  cleavage  would  seem  to 
require  change  in  chemical  composition  necessary  to  increase  the 
proportion  of  the  cleavage-making  minerals,  such  as  hornblende 
or  mica.  Chemical  evidence  seems  to  the  writer  to  point  this  way, 
though  it  is  not  yet  sufficient  for  proof. 

CLEAVAGE  IN  ITS  RELATIONS  TO  DIFFERENTIAL  PRESSURES 

It  has  been  shown  that  rock  cleavage  is  determined  by  the 
parallelism  of  mineral  constituents  and  that  this  parallelism  is 
developed  by  rock  flowage,  which  implies  differential  pressures. 
It  now  remains  to  discuss  the  attitude  of  cleavage  with  reference 
to  specific  pressure  conditions. 

What  experimental  evidence  there  is  indicates  that  in  a  non- 
rotational  strain  (see  page  16)  mineral  particles  tend  to  arrange 
themselves  with  their  longest  dimensions  normal  to  the  direction 
of  the  pressure.  There  is  practically  no  experimental  evidence 
bearing  on  the  arrangement  of  particles  under  rotational  strain  or 
shearing,  so  common  in  nature. 

Wright1  melted  about  50  grams  each  of  wollastonite,  diopside, 
and  anorthite,  and  plunged  the  melt  into  water,  thereby  forming  a 
glass.  Cubes  were  then  cut  from  these  glasses,  heated  to  a  viscous 
state  at  which  crystallization  first  begins,  and  subjected  to  vertical 
pressure.  Microscopic  examination  showed  that  the  three  minerals 
named  had  crystallized  with  their  longer  dimensional  axes  normal 
to  the  pressure. 

1  Wright,  F.  E.,  Schistosity  by  crystallization.  A  qualitative  proof:  Amer. 
Jour.  Sci.,  4th  ser.,  Vol.  22,  1906,  p.  226. 


FLOW   CLEAVAGE  85 

Becker  and  Day1  have  shown  that  although  crystals  are  able  to 
grow  in  a  given  direction  in  spite  of  contracting  forces,  their 
growth  in  the  plane  normal  to  the  pressure  is  vastly  greater, 
whether  this  be  the  normal  direction  of  elongation  due  to  habit  or 
not.  Ordinarily  in  schists  the  elongation  of  the  crystal  is  that  of 
its  normal  habit,  indicating  perhaps  that  the  crystals  favorably 
oriented  to  grow  with  normal  habit  have  grown  at  the  expense  of 
those  not  favorably  oriented. 

Relations  of  cleavage  to  strain: — Field  observations  have  to  do 
principally  with  the  relation  of  cleavage  to  rock  strain  (see  page  14), 
which  can  be  seen,  and  not  with  stress,  which  can  not  be  seen  and 
may  only  be  inferred  from  the  strain.  After  having  proved  the 
relation  of  cleavage  to  strain,  the  general  relations  of  strain  to 
stress  may  be  considered. 

It  seems  self-evident  that  the  longer  dimensions  of  mineral 
particles  in  a  cleavable  rock  lie  parallel  to  the  elongation  of  the 
rock  mass  developed  during  rock  flowage.  This  relationship  has 
been  so  generally  assumed  by  geologists  that  at  first  thought  it 
would  seem  entirely  superfluous  to  present  evidence  in  proof  of  it. 
But  it  has  been  questioned  by  able  geologists.  Becker  2  has  held 
that  the  elongation  of  the  rock  mass  may  be  inclined  to  the  common 
direction  of  the  major  axes  of  the  mineral  particles.  The  student, 
when  asked  how  he  knows  that  cleavage  is  parallel  to  rock  elonga- 
tion, is  often  completely  at  sea.  It  is  simply  a  matter  of  observa- 
tion to  determine  definitely  whether  the  cleavage  is  parallel  to  the 
elongation  of  the  mass  as  a  whole.  Evidence  indicating  this 
parallelism  is  as  follows:  (1)  Distortion  of  pebbles  of  a  conglomerate. 
Schistose  conglomerates  show  by  the  distortion  of  their  pebbles 
the  plane  of  elongation,  although  it  may  sometimes  be  difficult 
to  distinguish  the  shapes  of  undeformed  pebbles  from  those  of 
deformed  ones.  The  cleavage  of  the  matrix  is  approximately 
parallel  to  the  greater  diameters  of  the  flattened  pebbles,  although 
it  curves  somewhat  at  the  ends  of  the  pebbles.  (2)  Distortion  of 
mineral  crystals.  The  plane  of  cleavage  is  marked  by  mica  plates 
or  hornblende  crystals,  while  the  associated  quartz  and  feldspar 
particles  may  be  fractured  at  angles  with  the  plane  of  cleavage. 

1  Becker,  G.  F.,  and  Day,  A.  L.,  The  linear  force  of  growing  crystals:  Proc.  Wash. 
Acad.  Sci.,  Vol.  7,  1905,  pp.  283-288. 

2  Becker,  G.  F.,  Current  theories  of  slaty  cleavage:  Amer.  Jour.  Sci.,  4th  Ser., 
Vol.  24,  1907,  pp.  7-10. 


86  STRUCTURAL   GEOLOGY 

The  displacement  of  the  parts,  which  often  accompanies  such 
fractures,  is  observed  to  extend  the  fractured  parts  in  the  plane 
of  rock  cleavage.  (3)  Distortion  of  volcanic  textures.  The  original 
ellipsoidal  parting  of  basalts  frequently  shows  a  flattening,  with 
or  without  fracture;  in  such  cases  the  ellipsoids  and  the  matrix 
have  a  flow  cleavage  parallel  to  the  longer  diameters.  The  elonga- 
tion of  amygdules  and  spherulites  in  planes  parallel  to  the  rock 
cleavage  is  likewise  of  common  occurrence.  (4)  Distortion  of 
fossils.  The  elongation  of  fossils  in  the  plane  of  cleavage  has  been 
observed  in  cleavable  rocks.  (5)  Distortion  of  beds  and  attitude  of 
folds.  Folds  often  show  the  direction  of  shortening  of  the  deformed 
rock  mass.  (6)  Relations  to  intrusives.  Intrusions  of  great  masses 
of  igneous  rocks,  and  particularly  deep-seated  batholiths,  exert 
pressure  against  their  walls.  Any  cleavage  developed  in  the  sur- 
rounding rocks  is  parallel  to  the  periphery  of  the  intrusive  masses. 

It  is  concluded  then  that  the  longer  dimensions  of  mineral 
constituents  are  parallel  to  the  directions  or  planes  of  elongation 
of  the  rock  mass.  Thus  an  adequate  statement  of  the  relations 
of  rock  cleavage  to  the  stresses  which  have  produced  it  must  be  a 
statement  which  will  cover  the  various  ways  in  which  stress  has 
elongated  and  shortened  rock  masses. 

Relations  of  cleavage  to  stress: — In  the  simplest  possible  terms 
stress  has  been  effective  in  distorting  rock  masses  (1)  (see  pp.  16- 
21)  by  non-rotational  strain,  in  which  the  axes  of  stress  and  strain 
remain  mutually  constant  throughout  the  deformation,  and  (2)  by 
rotational  strain  in  which  there  is  a  continuous  change  in  the  posi- 
tion of  the  strain  axes  as  compared  with  the  stress  axes  during  the 
distortion.  In  the  first  case  the  elongation  of  the  rock  mass  is 
normal  to  the  greatest  stress  and  remains  so  through  the  deforma- 
tion; in  the  second  case  the  elongation  of  the  rock  mass  is  con- 
stantly changing  in  direction  with  reference  to  the  principal 
stress,  and  ultimately  the  elongation  may  be  considerably  inclined 
to  the  maximum  stress.  It  is  held  by  Hoskins1  that  at  any  instant 
the  tendency  for  elongation  is  approximately  normal  to  the  greatest 
stress,  but  that  the  rotational  tendency  results  in  inclining  the 
final  elongation  to  the  greatest  stress. 

Substituting  rock  cleavage  for  greatest  elongation  of  the  rock 

1  Hoskins,  L.  M.,  Flow  and  Fracture  of  rocks  as  related  to  structure:  16th  Ann. 
Kept.  U.  S.  G.  S.,  Pt.  I,  1896,  pp.  845-874. 


FLOW   CLEAVAGE  87 

mass,  the  statement  of  the  relations  of  cleavage  to  pressure  is  as 
follows:  In  a  non-rotational  strain  cleavage  is  developed  normal 
to  the  greatest  stress;  in  rotational  strain,  while  at  any  instant 
there  may  be  a  tendency  for  it  to  be  developed  normal  to  the 
greatest  stress,  there  is  here  a  rotational  element  which  brings  it 
into  position  inclined  to  the  greatest  stress.  All  distortional 
strains  in  rock  masses  belong  to  these  two  classes,  rotational  and 
non-rotational,  and  usually  to  some  combination  of  the  two. 
Cleavage,  therefore,  is  developed  under  some  combination  of 
rotational  and  non-rotational  strains  and  may  be  said  to  be  pro- 
duced both  normal  and  inclined  to  pressures. 

Specific  inferences  from  field  observations  as  to  the  pressure 
conditions  controlling  cleavage  are  discussed  on  pp.  119  and  128 
in  connection  with  folds. 


GNEISSIC  STRUCTURE 

Gneissic  structure  means  a  banding  of  constituents,  of  which 
feldspar  is  important,  with  or  without  the  parallel  dimensional 
arrangement  necessary  for  rock  cleavage.  A  schist  always  has  a 
parallel  dimensional  arrangement  and  may  or  may  not  contain 
feldspar.  A  gneiss  may  or  may  not  have  a  parallel  arrangement, 
but  always  has  a  banding  and  contains  feldspar.  So  far  as  this 
parallel  arrangement  is  present,  gneissic  structure  has  been  dis- 
cussed under  the  heading  of  rock  cleavage.  In  many  cases,  how- 
ever, cleavage  in  gneisses  is  not  good.  The  essential  mineralogical 
difference  between  gneisses  and  schists  is  the  possession  by  the 
gneisses  of  a  relatively  small  amount  of  the  platy  and  columnar 
minerals  so  necessary  for  a  good  rock  cleavage,  and  correspond- 
ingly more  feldspar  and  quartz. 

The  origin  of  perhaps  the  majority  of  gneisses  is  not  yet  known. 
In  a  few  instances  the  structure  has  been  identified  as  an  original 
magmatic  flow  structure,  the  "protoclastic"  structure.  In  other 
cases  it  is  the  result  of  secondary  rock  flowage,  either  of  igneous  or 
sedimentary  rocks,  the  "  crystalloblastic"  structure.  Some  criteria 
which  have  been  useful  in  discriminating  the  igneous  and  sedi- 
mentary gneisses  resulting  from  rock  flowage  are  the  broader 
field  relations,  the  chemical  composition,  the  possession  of  igneous 
or  crystalloblastic  textures,  and  the  content  and  form  of  the  heavy 


88 


STRUCTURAL   GEOLOGY 


FIG.  42.  Photomicrographs    showing   the   progressive    granulation   of   the    Morin 
anorthosite  under  the  influence  of  pressure.    After  Adams. 


GNEISSIC   STRUCTURE 


89 


residues  such  as  zircon.1  These  criteria  may  often  be  decisive  when 
applied  collectively,  but  seldom  when  used  separately.  They  are 
discussed  on  a  subsequent  page  (97) . 

Gneisses  have  been  known  to  develop  by  rock  flowage  from 
rocks  which  under  other  conditions  have  yielded  schists.     What, 


FIG.  43.  Photomicrograph  of  leaf  gneiss  from  the  Laurcntian  area  north  of  Mon- 
treal. Slide  furnished  by  Frank  D.  Adams.  Doctor  Adams  has  described  the 
leaf  gneiss  as  resulting  from  granulation  of  a  hornblende  granite,  all  stages  of 
the  process  having  been  noted.  (See  Part  J  of  Vol.  VIII  of  the  Geological 
Survey  of  Canada,  1895.)  The  striated  feldspars  have  irregular  angular  shapes 
such  as  characteristically  result  from  granulation.  The  two  bands  of  quartz 
crossing  the  slide  evidently  owe  their  form  and  arrangement  finally  to  recrystal- 
lization,  although  granulation  may  have  been  an  important  initial  process. 
It  will  be  noted  that  the  quartz  individuals  have  dimensional  but  not  crystal- 
lographic  parallelism. 

then,  are  the  conditions  which  determine  whether  the  gneissic 
or  the  schistose  structure  will  result  from  the  rock  in  question? 
Study  of  all  analyses  available  of  schistose  and  gneissic  rocks 

1  Trueman,  J.  D.,  The  value  of  certain  criteria  for  the  determination  of  the  origin 
of  foliated  crystalline  rocks:  Jour.  Geol.,  Vol.  20,  1912,  pp.  228-258,  300-315. 


90  STRUCTURAL   GEOLOGY 

indicates  a  higher  percentage  of  moisture  for  the  schists  than 
for  the  gneisses.  The  moisture  is  concentrated  largely  in  the 
tabular  and  columnar  minerals  which  so  largely  determine  rock 
cleavage.  Many  schists  are  found  along  shearing  planes  in  non- 
schistose  types.  Water  has  been  allowed  access  here  by  means 
of  the  fractures.  Other  schists  represent  anamorphosed  sedi- 
ments which  originally  contained  a  good  deal  of  water.  The 
principal  change  in  the  development  of  secondary  gneissic  struc- 
ture is  one  of  granulation  and  recrystallization  of  substances 
present,  not  the  development  of  new  tabular  or  columnar  minerals 
requiring  water.  A  good  illustration  is  the  case  of  the  sheared 
anorthosites,  or  gneisses,  described  by  Adams  l  as  developing  from 
fresh  anorthosites  entirely  by  granulation  accompanied  by  a 
minimum  of  recrystallization  and  consequent  development  of 
hornblende  and  mica.  (See  Fig.  42.)  Adams  also  has  experimen- 
tally deformed  diabase,  under  dry  conditions  unfavorable  for 
recrystallization.  The  deformation  was  principally  by  granula- 
tion. The  result  was  a  gneiss. 

If  water  is  essential  to  the  development  of  the  best  cleavage- 
giving  minerals,  it  may  be  argued  that  its  absence  may  be  respon- 
sible for  the  lack  of  development  of  a  good  cleavage  during  rock 
flowage.  Although  it  is  not  proved,  it  seems  entirely  plausible  that 
many  of  the  gneisses,  especially  if  they  developed  from  granite, 
have  been  formed  under  deep-seated  conditions  unfavorable  either 
to  the  original  presence  of  water  or  to  its  introduction  during 
deformation.  There  may  be  other  and  more  decisive  factors,  as 
yet  unknown. 

IDIOMORPHIC  OR  PORPHYRITIC  TEXTURES  DEVEL- 
OPED BY  ROCK   FLOWAGE 

Garnet,  staurolite,  tourmaline,  andalusite,  chloritoid,  and 
other  heavy  anhydrous  minerals  of  this  kind  are  uniformly  idio- 
morphic  or  porphyritic  in  cleavable  rocks.  They  develop  by 
recrystallization  after  rock  flowage  has  ceased,  but  probably 
while  the  rock  is  still  under  high  pressure  and  temperature,  as 
is  evidenced  by  their  high  specific  gravity  and  characteristic 

1  Adams,  F.  D.,  Report  on  the  geology  of  a  portion  of  the  Laurentian  area  lying 
to  the  north  of  the  island  of  Montreal;  Ann.  Rept.  Geol.  Survey  of  Canada,  Vol.  8, 
pt.  J,  1896,  p.  85  et  seq. 


PORPHYRITIC   TEXTURE 


91 


occurrence  in  the  proximity  of  intrusive  igneous  rocks.  Their 
late  development  by  recrystallization  is  shown  by  the  following 
considerations:  (1)  They  appear  in  rocks  clearly  derived  by  rock 
flowage  from  others  originally  lacking  such  minerals.  (2)  They 
frequently  lie  at  large  angles  to  the  prevailing  cleavage  in  the 
rock.  (3)  They  do  not  show  the  degree  of  mechanical  deforma- 
tion that  they  would  necessarily  have  possessed  had  they  devel- 
oped in  their  present  positions  before  flowage  had  ceased.  Many 


FIG.  44.  Photomicrograph  of  chloritoid  crystal  in  micaceous  and  quartzose  schist 
from  Black  Hills.  The  chloritoid  crystal  here  shown  has  developed  later  than 
the  rock  flowage  producing  the  prevailing  cleavage  of  the  rock.  The  chloritoid 
has  grown  at  the  expense  of  the  other  constituents  of  the  rock,  using  all  the 
material  necessary  for  its  growth  and  leaving  the  excess  of  material  in  the  form 
of  inclusions,  which  retain  their  dimensional  parallelism  with  the  prevailing 
rock  cleavage. 

of  the  crystals  are  long  and  acicular,  and  would  surely  have  been 
broken  if  any  considerable  movement  had  occurred  subsequent 
to  their  development.  (4)  They  include,  within  their  boundaries, 
minerals  in  part  similar  to  those  in  the  remainder  of  the  rock, 
and  which  have  an  arrangement  of  their  greater  diameters  in  the 


92  STRUCTURAL   GEOLOGY 

plane  of  rock  cleavage,  showing  that  to  some  degree  at  least  they 
were  formed  during  rock  flowage.  (5)  The  mica  and  the  other 
constituents  of  cleavable  rocks,  which  are  certainly  developed 
by  recrystallization  during  the  process  of  deformation,  are  fre- 
quently seen  to  end  abruptly  at  the  periphery  of  a  mineral  of  this 
group  and  not  to  curve  around  it  as  they  often  do  about  the 
resistant  minerals  in  schists.  If  the  rock  had  flowed  after  the 
formation  of  the  porphyritic  crystals,  crowding  and  bending  of  the 
micas  must  inevitably  have  occurred.  (6)  The  usual  large  size 
of  minerals  of  this  group,  as  compared  with  their  associated  mineral 
particles,  suggests  their  development  subsequent  to  rock  flowage, 
when  granulation  is  no  longer  tending  to  break  down  the  crystals. 

While  the  development  of  this  group  of  crystals  is  believed  to 
have  been  mainly  later  than  the  formation  of  the  cleavage,  it  is 
true  also  that  in  some  cases  subsequent  flowage  has  resulted  in 
their  being  fractured  and  crowding  the  other  constituents.  The 
very  fact  that  the  effects  of  further  movement  are  so  conspicuous 
confirms  the  conclusion  that  the  secondary  porphyritic  minerals 
not  showing  these  effects  developed  after  the  movement  ceased. 

We  may  only  speculate  as  to  the  conditions  of  this  peculiar 
development  of  non-arranged  minerals.  They  are  probably  high 
temperature  and  pressure,  but  apparently  no  differential  stress, 
requiring  movement.  If  the  pressure  and  temperature  may  be 
considered  as  having  become  so  great  as  to  develop  hydrostatic 
conditions,  there  would  be  no  differential  pressures  necessary 
for  a  parallel  arrangement  of  constituents  and  this  might  afford  a 
plausible  explanation  of  the  development  of  these  non-oriented 
porphyritic  minerals. 

ROCK  FLOWAGE  WITHOUT  RETENTION  OF  CLEAVAGE 

Marble  is  the  commonest  example  of  a  rock  which  undergoes 
flowage  without  retaining  cleavage.  It  often  occurs  between 
schistose  beds  which  have  flowed,  without  doubt  the  marble  itself 
has  flowed,  and  yet  it  possesses  no  cleavage.  Cleavage  may  be 
produced  experimentally  in  marble  by  pressure  alone,  when  the 
conditions  are  not  favorable  for  recrystallization.1  Microscopic 

1  Adams,  F.  D.,  and  Nicolson,  J.  T.,  An  experimental  investigation  into  the  flow 
of  marble:  Phil.  Trans.  Roy.  Soc.  of  London,  Vol.  195,  1901,  pp.  363-401.  See  also 
Adams,  F.  D.,  and  Coker,  E.  G.,  The  flow  of  marble:  Amer.  Jour.  Sci.,  Vol.  29, 
1910,  pp.  465-487. 


FLOWAGE   WITHOUT   CLEAVAGE  93 

examination  indicates  that  this  has  been  accomplished  by  gran- 
ulation, slicing,  and  gliding  of  the  calcite  crystals.  Rarely  such  a 
cleavage  is  observed  in  marble  deformed  under  natural  conditions. 
It  may  be  supposed  that  many  marbles  have  shown  this  structure 
in  the  early  stages  of  their  flowage,  but  calcite  recrystallizes  so 
easily  that  the  parallel  structure  caused  by  mechanical  deformation 
is  soon  destroyed.  The  recrystallized  calcite  crystals  do  not  have 
the  habit  necessary  for  a  good  dimensional  arrangement  in  schists. 
So  far  as  the  limestones  have  impurities  in  them,  secondary 
silicates  are  likely  to  develop,  such  as  actinolite  and  tremolite, 
which  by  their  arrangement  may  give  the  rock  a  cleavage. 

OBLITERATION   OF  TEXTURES  BY    ROCK    FLOWAGE 

Recrystallization,  the  dominant  process  in  rock  flowage,  tends 
toward  an  increase  in  the  size  of  grain,  the  segregation  of  minerals 
into  bands,  a  uniformity  in  size  and  shape  of  the  mineral  particles, 
and  the  growth  of  new  minerals  such  as  mica  or  hornblende  not 
previously  existent  in  the  rock.  Previous  textures  are  commonly 
destroyed.  Bedding  is  locally  not  completely  obliterated,  because 
alternation  of  beds  of  originally  different  mineralogic  character 
and  texture  determines  to  some  extent  the  kinds  and  size  of  the 
secondary  mineral  particles  formed  in  these  beds  by  rock  flowage. 
Thus  a  faint  banding  of  dark  or  light  minerals  or  of  fine  or  coarse 
minerals  may  mark  the  original  bedding  in  a  schistose  rock.  (See 
Figs.  45,  46  and  47). 


94 


STRUCTURAL   GEOLOGY 


FIG.  45.  Photomicrograph  of  micaceous  and  quartzose  schist  with  cleavage  de- 
veloped across  original  bedding,  from  Little  Falls,  Minn.  A  graywacke-slate, 
in  which  the  banding  has  been  marked  by  difference  in  texture  as  well  as  in 
composition,  has  been  subjected  to  deformation,  with  the  result  that  a  cleavage 
has  been  superposed  at  right  angles  to  the  original  bedding.  Originally  the 
longer  diameters  of  the  particles  of  the  bedded  rock  were  parallel  to  the  bed- 
ding. Accompanying  the  development  of  flow  cleavage  most  of  the  con- 
stituents of  the  rock  have  been  recrystallized.  The  quartz  particles  shown  in 
the  light  band  have  been  drawn  out  with  their  longer  diameters  nearly  at  right 
angles  to  the  former  plane  of  their  longer  diameters,  and  abundant  new  mica 
has  developed  with  its  greater  diameters  and  mineral  cleavage  normal  to  the 
plane  of  bedding. 


FLOW  CLEAVAGE  AND   BEDDING 


95 


FIG.  46.  Cleavage  crossing  bedding  of  slates,  St.  Louis  river,  Minnesota.  The 
broad  plane  surface  dipping  to  the  right  is  a  bedding  plane.  The  structure 
dipping  more  steeply  to  the  right  is  cleavage. 


96 


STRUCTURAL   GEOLOGY 


FIG.  47.  Slaty  structure  and  its  relation  to  bedding  planes.     Two  miles  south  of 
Walland,  Tenn.    After  Keith. 


ROCK   FLOWAGE  97 

*i  '*'  '  "''" 

"  IDENTIFICATION   OF   SCHISTS  AND   GNEISSES 

Not  only  does  rock  flowage  tend  to  obliterate  primary  textures 
but  it  modifies  the  chemical  and  mineralogical  composition. 

In  proportion,  then,  as  rocks  have  undergone  rock  flowage,  there 
may  be  difficulty  in  ascertaining  their  origin.  The  identification  of 
the  origin  of  schists  and  gneisses  is  more  largely  a  metamorphic 
than  a  structural  problem,  but  it  is  difficult  to  separate  the  two 
phases  of  the  problem.  Both  are  covered  in  the  following  sum- 
mary of  criteria. 

FIELD  RELATION  AS  A  MEANS  OF  IDENTIFYING  SCHISTS  AND 

GNEISSES 

Field  and  microscopic  observation  of  gradations  from  unde- 
formed  rocks  into  schists  or  slates  gives  certain  empirical  methods 
for  recognition  of  origin  of  some  schists  and  gneisses.  ..For  instance, 
a  shale  alters  to  a  slate  and  this  in  turn  to  a  phyllite.  While  it  is 
difficult  from  the  study  of  the  phyllite  alone  to  determine  its 
origin,  it  so  often  has  been  observed  as  the  end-product  of  this 
series  of  changes  that  there  is  little  danger  of  mistake  if  it  is  re- 
ferred back  to  a  shale  or  a  mud.  A  sandstone  or  quart zite  may  be 
traced  into  a  mica-quartz-schist,  seldom  into  a  hornblende  schist. 
A  similar  schist  may  be  derived  from  the  secondary  deformation 
of  certain  acid  igneous  rocks.  A  quartz-mica-schist  therefore  is 
regarded  as  the  natural  development  of  an  acid  rock,  but  whether 
sedimentary  or  igneous  may  be  doubtful,  when  field  relations  do  not 
decide.  A  basalt  is  observed  to  grade  into  a  chloritic  and  micaceous 
schist.  The  same  result  may  be  observed  where  certain  shales 
are  altered.  Basic  igneous  rock  (especially  in  the  vicinity  of 
intrusives)  by  rock  flowage  may  pass  into  coarsely  crystalline 
hornblende  schists  or  gneisses.  Amphibolites  are  known  to  be 
formed  also  by  alteration  of  limestone.  Some  banded  gneisses,  by 
their  association  with,  and  gradation  to,  granites,  and  by  their 
mineralogical  composition,  seem  to  be  surely  the  result  of  rock 
flowage  of  a  granite,  though  cases  of  proved  gradation  are  rare. 
It  has  been  observed,  however,  that  certain  sediments,  such  as  an 
impure  quartz  sand,  have  gone  over  to  gneisses  with  general  as- 
pects similar  to  those  presumably  developed  from  a  granite.  The 
passage  of  a  dolomite  into  a  talc  schist  is  not  uncommon. 


98  STRUCTURAL   GEOLOGY 

Schists  or  gneisses  may  be  interbedded  with  sediments  and 
be  themselves  in  beds  strongly  suggestive  of  sedimentary  origin. 
They  may  be  in  an  igneous  complex  and  have  irregularity  of  form 
or  distribution  or  relations  to  adjacent  rocks  more  characteristic 
of  an  igneous  mass  than  of  sedimentary  beds.  Some  gneisses  of 
the  Laurentian  are  clearly  original  igneous  rocks  intrusive  into 
adjacent  rocks.  Where  the  schist  or  gneiss  shows  marked  differ- 
ences in  composition  in  different  beds  or  bands,  and  this  composi- 
tion is  persistent  throughout  these  bands  for  long  distances,  it  is 
suggestive  of  sedimentary  origin,  especially  if  some  of  the  beds  have 
mineral  or  chemical  composition  of  sediments.  The  Baltimore 
and  Carolina  gneisses  of  the  Piedmont  Plateau,1  and  the  Idaho 
Springs  formation  of  the  Georgetown  area  of  Colorado2  are  of  this 
type.  Yet  analogous  structure  has  been  produced,  perhaps  on  a 
smaller  scale,  by  injections  of  igneous  masses  along  parallel  planes. 

On  the  whole,  with  our  present  knowledge,  field  observations 
are  likely  to  yield  more  satisfactory  conclusions  as  to  origin  than 
other  criteria  below  discussed. 

MINERAL  COMPOSITION  AS  A  MEANS  OF  IDENTIFYING  SCHISTS 

AND   GNEISSES 

A  great  preponderance  of  quartz  is  perhaps  more  often  charac- 
teristic of  a  sedimentary  than  an  igneous  rock.  Where  a  gneiss 
or  schist  is  dominantly  quartz,  one  looks  for  other  evidences  of 
sedimentary  origin.  But  the  existence  of  highly  quartzose  rocks  of 
the  pegmatite  and  alaskite  types  makes  quartz  content  alone  a 
doubtful  criterion.  Preponderance  of  calcite  is  more  satisfactory 
evidence  of  sedimentary  origin. 

The  abundant  development  of  aluminum  silicate  minerals 
such  as  staurolite  and  sillimanite3  has  been  more  commonly  ob- 

1  See:  Keith,  Arthur,  Washington  folio  (No.  70),  Geol.  Atlas  U.  S.,  U.  S.  Geol. 
Survey,  1900. 

Mathews,  E.  B.,  Correlation  of  Maryland  and  Pennsylvania  Piedmont  forma- 
tions: Bull.  Geol.  Soc.  Am.,  Vol.  16,  1905,  pp.  329-346. 

Bascom,  F.,  Piedmont  district  of  Pennsylvania:  Bull.  Geol.  Soc.  Am.,  Vol.  16, 
1905,  pp.  289-328. 

2  Spurr,  J.  E.,  and  Garrey,  G.  H.,  Economic  geology  of  the  Georgetown  quad- 
rangle, Colorado:  Prof.  Paper  U.  S.  Geol.  Survey  No.  63,  1908,  p.  44. 

3  Emmons,  W.  H.,  and  Laney,  F.  B.,  Preliminary  report  on  the  mineral  deposits 
of  Ducktown,  Tenn.:  Bull.  470,  U.  S.  Geol.  Survey,  1911,  p.  158. 

Spurr,  J.  E.,  and  Garrey,  G.  H.,  Economic  geology  of  the  Georgetown  quad- 
rangle, Colorado:  Prof.  Paper  U.  S.  Geol.  Survey  No.  63,  1908,  p.  44. 


IDENTIFICATION   OF  SCHISTS  AND   GNEISSES    99 

served  in  metamorphosed  sediments  than  in  igneous  rocks.  Any 
of  these  minerals,  however,  may  be  found  also  in  igneous  rocks. 

Where  gneiss  is  strongly  feldspathic,  it  is  not  likely  to  be  re- 
garded as  of  sedimentary  origin.  Yet  so  far  as  the  sediment  is 
undecomposed,  it  may  be  largely  feldspathic,  and  also  the  anamor- 
phism of  a  nonfeldspathic  sediment  might  make  it  feldspathic, 
though  it  is  a  question  whether  to  a  degree  common  to  many 
gneisses. 

The  presence  of  graphite  disseminated  evenly  through  a  band 
or  zone  becomes  presumptive  evidence  of  sedimentary  origin, 
especially  where,  as  in  the  Adirondack  graphites,  there  are  other 
evidences  present.1  Some  graphite  may  be  igneous  in  origin,  but 
when  evenly  distributed  in  amount  up  to  about  6%  in  a  generally 
slaty  or  quartzose  zone,  the  hypothesis  of  igneous  origin  becomes 
untenable. 

Mica  or  chlorite  or  hornblende  affords  no  satisfactory  criterion 
of  identification  of  origin,  for  these  minerals  develop  both  from 
sedimentary  and  from  igneous  rocks.  But  so  far  as  present  evi- 
dence goes,  they  seem  to  develop  more  readily  from  sediments 
than  from  igneous  rocks,  perhaps  because  water  is  necessary. 
This  criterion  must  be  most  carefully  used,  in  view  of  the  fact 
that  sedimentary  composition  may  be  approached  by  the  weather- 
ing of  igneous  rocks  prior  to  anamorphism.  The  basalts  of  the 
Menominee  district  described  by  George  H.  Williams  alter  by 
katamorphism  into  chloritic  rocks  and  under  pressure  alter  to 
chlorite-schists.  The  mineral  change  from  the  fresh  rock  is  the 
same  in  both  cases.  It  may  be  that  the  chlorite-schist  was  pre- 
ceded by  katamorphism  of  the  basalt. 

The  separation  of  minute  accessory  constituents  by  washing  is  a 
means  for  identifying  origin  which  has  not  yet  been  sufficiently 
used.  In  deeply  weathered  rocks  like  those  of  central  Brazil  this 
method  has  been  used  effectively  by  Dr.  Derby  and  associates  in 
determining  whether  the  weathered  material  is  igneous  or  sedimen- 
tary.2 Minerals  of  igneous  rocks  like  monazite,  zircon,  sphene, 
garnet,  and  so  on,  are  remarkably  resistant  to  weathering,  and  will 
remain  in  well  defined  crystals  when  all  the  other  constituents  have 

1  Bastin,  E.  S.,  Origin  of  certain  Adirondack  graphite  deposits:  Econ.  GeoL, 
Vol.  5,  1910,  pp.  134-157. 

2  Derby,  O.  A.,  On  the  separation  and  study  of  the  heavy  accessories  of  rocks: 
Proc.  Rochester  Acad.  Sci.,  Vol.  1,  1891,  pp.  198-206. 


100  STRUCTURAL   GEOLOGY 

altered.  When  these  are  found  unmodified  in  the  weathered  rocks, 
it  is  assumed  that  the  reck  is  of  igneous  origin.  The  argillaceous 
sediments  lack  these  substances.  Quartzites  may  possess  them, 
but  they  are  there  likely  to  show  distinct  wearing  by  attrition. 
In  the  schistose  equivalent  of  the  quartzite  the  rounded  grains 
persist,  particularly  in  zircon.  Where,  therefore,  in  an  argillaceous 
schist  these  heavy  accessory  minerals  are  lacking  or  in  a  quartz 
schist  are  rounded,  a  sedimentary  origin  is  probable.1 

CHEMICAL  COMPOSITION  AS  A  MEANS  OF  IDENTIFYING  IGNE- 
OUS OR  SEDIMENTARY  ORIGIN  OF  GNEISSES  AND  SCHISTS 

If  the  composition  of  a  schist  or  gneiss  is  substantially  that  of  an 
igneous  rock,  their  igneous  origin  is  usually  regarded  as  probable 
at  first  thought,  yet  the  basis  for  this  supposition  is  an  unsatis- 
factory one,  for  so  far  as  sediments  are  produced  from  igneous 
rocks  by  disintegration  rather  than  decomposition,  the  primary 
composition  of  the  sediments  approaches  that  of  the  igneous 
rocks.  Also  facts  have  been  found  to  show  that  the  general  tend- 
ency of  anamorphism  of  sediments  is  toward  the  reproduction  of 
the  composition  of  igneous  rocks,  both  by  dynamic  and  contact 
metamorphism.  The  tendency  is  not  known  fully  to  accomplish 
this  result,  but  certainly  it  approaches  it  closely  enough  to  give  a 
composition  which  is  not  so  different  from  that  of  the  igneous  rock 
that  it  may  be  certainly  classed  as  sedimentary.  So  far  as  quanti- 
tative evidence  yet  goes,  igneous  composition  of  a  schist  may 
indicate  igneous  origin,  it  may  indicate  that  the  schist  came  from  a 
sediment  of  igneous  composition,  or  it  may  represent  an  extreme 
of  anamorphism  of  sediments  which  has  tended  to  reproduce  igne- 
ous composition  in  them. 

If  the  composition  of  the  schist  or  gneiss  is  that  of  a  sedimentary 
rock,  it  has  been  somewhat  generally  assumed  that  this  proves  the 
sedimentary  origin  of  the  schist  or  gneiss.2  Distinctive  features 
of  sedimentary  origin,  as  summarized  by  Bastin,3  are  dominance  of 

1  Trueman,  J.  D.,  The  value  of  certain  criteria  for  the  determination  of  the  origin 
of  foliated  crystalline  rocks:  Jour.  Geol.,  Vol.  20,  1912,  pp.  244-258. 

2  Adams,  F.  D.,  Geology  of  a  portion  of  the  Laurentian  area  lying  to  the  north 
of  the  island  of  Montreal:  Ann.  Rept.,  Geol.  Survey  of  Canada,  Vol.  8,  part  J,  1896, 
p.  57  et  seq. 

Adams,  F.  D.,  and  Barlow,  A.  E.,  Geology  of  the  Hail  burton  and  Bancroft  areas, 
Ontario:  Geol.  Survey  Can.,  Mem.  No.  6,  1910. 

3  Bastin,  Edson  S.,  Chemical  composition  as  a  criterion  in  identifying  meta- 
morphosed sediments:  Jour.  Geol.,  Vol.  17,  1909,  p.  472. 


IDENTIFICATION   OF   SCHISTS   AND   GNEISSES    101 

magnesia  over  lime,  of  potassa  over  soda,  excess  of  alumina,  and 
high  silica.  To  these  must  be  added  all  other  known  chemical 
peculiarities  of  sediments.  These  criteria  should  be  used  with 
knowledge  and  consideration  of  the  general  chemical  processes 
involved  in  the  development  of  sediments.  However,  so  far  as  an 
igneous  rock  is  katamorphosed  before  or  after  it  becomes  schistose, 
its  composition  approaches  that  of  a  sediment,  in  which  case  the 
composition  might  be  that  of  a  sediment  and  yet  the  rock  may 
never  have  been  a  sediment.  Bastin  recognizes  this  possibility, 
but  considers  it  of  minor  significance.  Some  schists  and  gneisses 
develop  from  igneous  rocks  and  retain  original  igneous  composi- 
tion. It  is  known  that  others  do  not.  It  has  not  yet  been  proved 
which  is  the  common  case,  but  quantitative  evidence  is  less  satis- 
factory for  the  former  than  for  the  latter.  Plutonic  rocks  may  be 
less  katamorphosed  than  volcanics  prior  to  anamorphism,  but 
direct  evidence  of  this  is  not  available.  In  addition  to  surface 
weathering  it  is  necessary  to  include  all  hydration  and  solution 
which  may  take  place  in  the  zone  of  fracture,  and  also  hydrother- 
mal  alteration  which  has  essentially  the  same  chemical  effect  as 
weathering  as  far  as  lime-magnesia  and  soda-potassa  ratios  are 
concerned.  The  conclusion  that  a  sedimentary  composition  of  a 
gneiss  or  schist  means  sedimentary  origin  is  based  simply  on  the 
fact  that  some  igneous  rocks  become  schistose  or  gneissic  without 
change  in  composition  and  ignores  the  equally  well  established  fact 
that  others  have  approached  the  sedimentary  rocks  in  composition 
prior  to  or  during  or  after  the  alteration  to  schist  or  gneiss.  Appli- 
cation of  the  chemical  criteria  for  sedimentary  origin  outlined  by 
Bastin  to  the  green  schists  of  the  Menominee  district  of  Michigan, 
shown  by  Williams1  and  others  to  be  largely  schistose  basalts, 
illustrates  the  uncertainty  of  these  criteria  in  determining  origin. 
Under  these  criteria,  part  of  Williams'  analyses  are  those  of  sedi- 
ments, part  are  those  of  igneous  rocks,  and  part  have  intermediate 
characters. 

Chemical  composition,  therefore,  in  the  present  state  of  knowl- 
edge, must  be  regarded  as  an  extremely  uncertain  basis  for  deter- 
mining igneous  or  sedimentary  origin.  If  the  composition  is  that 
of  an  igneous  rock,  it  is  plausible  to  assume  that  the  probability 

1  Williams,  G.  H.,  The  greenstone  schist  areas  of  the  Menominee  and  Marquette 
regions  of  Michigan:  Bull.  62,  U.  S.  Geol.  Survey,  1890. 


102  STRUCTURAL   GEOLOGY 

slightly  favors  igneous  origin,  but  the  same  composition  may  be 
possessed  by  a  sedimentary  rock,  either  because  of  its  primary 
character  or  because  of  composition  which  has  been  induced  in  it  by 
anamorphism.  If  the  composition  of  the  schist  or  gneiss  is  that  of 
a  sedimentary  rock,  the  balance  of  probability  would  perhaps 
slightly  favor  its  sedimentary  origin,  but  igneous  rocks  are  known 
also  to  take  on  this  composition,  either  prior  to  or  during  their 
anamorphism.  When  vastly  more  chemical  analyses  of  well 
selected  sets  of  rocks  become  available  to  show  specifically  the 
range  of  chemical  changes  in  anamorphism  of  both  igneous  and 
sedimentary  rocks,  it  may  be  possible  to  use  chemical  criteria 
which  will  aid  in  determining  the  origin  of  the  schists  and  gneisses. 

CONCLUSION  AS  TO  METHODS  OF  IDENTIFYING  GNEISSES  AND 

SCHISTS 

The  writer  knows  of  no  case  where  all  the  evidences  above  cited 
have  been  used  in  the  determination  of  sedimentary  origin  of  a 
gneiss.  As  one  surveys  the  methods  used  in  the  conclusions 
reached  in  various  investigations  of  gneisses  and  schists,  it  be- 
comes apparent  that  no  one  criterion  is  sufficient  to  establish 
sedimentary  origin. 

Gneisses  developed  secondarily  from  igneous  rocks  by  pressure 
and  recrystallization  have  been  positively  identified  in  even  fewer 
cases  than  sedimentary  gneisses.  Many  gneisses  are  known  to  be 
original  igneous  rocks  with  flow  structure;  a  few  have  been  found  to 
be  the  result  of  mechanical  breaking  down  by  granulation,  for 
instance,  the  granulated  anorthosites  described  by  Adams.1 
Many  gneisses  have  been  described  as  the  foliated  equivalents  of 
granites  as  result  of  pressure  and  recrystallization,  but  often 
without  adequate  proof  of  this  relation.  Lehmann  has  appar- 
ently shown  the  development  of  gneisses  from  granites  in  the 
Saxony  area.  In  parts  of  the  Lake  Superior  country  there  are 
gneisses  which  seem  to  have  such  relations  to  granite  gneisses  as 
would  result  from  secondary  pressures  and  recrystallization,  but 
there  is  not  a  single  proved  case  there.  Many  pairs  of  analyses  of 
granites  and  equivalent  gneisses  have  been  published,  but  these 

1  Adams,  F.  D.,  Report  on  the  geology  of  a  portion  of  the  Laurentian  area  lying 
to  the  north  of  the  island  of  Montreal:  Ann.  Kept.  Geol.  Survey  Can.,  Vol.  8,  part 
J,  1896,  p.  85  et  seq. 


IDENTIFICATION   OF   SCHISTS   AND   GNEISSES     103 

have  usually  been  made  on  the  assumption  that  the  gneiss  was  the 
result  of  secondary  alteration  of  granite  and  without  adequate 
consideration  of  the  possibility  that  gneissose  structure  may  be 
an  original  flow  structure. 

Many  more  schists  than  gneisses  have  been  proved  to  be  the 
result  of  mashing  of  igneous  rocks,  for  instance,  the  chlorite  schists 
so  commonly  developed  from  the  mashing  of  basalt,  illustrated  by 
the  schists  in  the  Keewatin  series  of  the  Lake  Superior  country; 
the  hornblende  schists  formed  in  these  rocks  by  contact  metamor- 
phism  of  granites;  micaceous  schists  formed  in  granites  and  por- 
phyries along  a  shear  zone.  In  fact,  so  commonly  do  the  igneous 
rocks  appear  when  mashed  to  take  on  schistose  as  contrasted  with 
gneissic  structure  as  to  raise  the  question  whether  this  is  not  the 
common  result  of  mashing  and  whether  gneisses  are  not  exceptional 
results,  most  gneisses  to  be  explained  as  igneous  rocks  with  original 
flow  structures. 

This  brings  us  back  to  a  suggestion  made  on  an  earlier  page, 
that  when  igneous  rocks  break  down  by  mashing,  there  tend  to 
develop  the  platy  and  columnar  hydrous  minerals  characteristic 
of  schists.  These  minerals  are  the  same  in  kind  as  those  derived 
from  the  anamorphism  of  a  sediment.  As  compared  with  the 
igneous  rock,  the  change  to  a  schist  amounts  to  katamorphism,  and 
requires  the  introduction  of  water  and  carbon  dioxide.  To  what- 
ever extent  gneiss  may  be  formed  by  the  mashing  of  igneous  rocks, 
and,  as  noted,  this  extent  is  extremely  problematic,  conditions 
different  from  those  forming  schists  are  implied  by  the  fact  that  the 
gneisses  have  relatively  less  amounts  of  platy  and  columnar  min- 
erals and  the  change  has  obviously  been  under  conditions  not 
those  of  hydration  and  carbonation,  or  katamorphism  in  general. 
We  have  suggested  that  gneisses  may  form  only  in  places  where 
the  agents  of  hydration  and  carbonation  are  lacking,  and  that 
where  these  agents  were  present,  the  change  is  more  toward  the 
schist  type. 

The  terms  " schist"  and  " gneiss"  have  been  used  as  representing 
two  contrasting  types  of  rocks.  It  is  of  course  to  be  recognized 
that  there  are  complete  gradations  between  schist  and  gneiss;  that 
it  probably  follows  therefore  that  there  are  many  conditions  of 
origin  of  the  schists  and  gneisses  from  igneous  rocks  intermediate 
between  those  described. 


STRUCTURES  COMMON  TO  BOTH  FRACTURE 
AND  FLOW 

FOLDS 

ELEMENTS  OF  FOLDS 

The  elements  of  a  simple  fold  are  indicated  in  the  following 
diagram  (Fig.  48)  taken  from  Willis. 

The  attitude  of  a  rock  bed  is  described  in  terms  of  strike  and  dip. 
Strike  is  the  direction  of  line  of  intersection  of  the  bed  with  the 
horizontal;  dip  is  the  angle  between  the  bed  and  the  horizontal, 


FIG.  48.  Parts  of  folds.    After  Willis. 

measured  at  right  angles  to  the  strike.    Folds  are  usually  deter- 
mined by  the  correlation  of  strike  and  dip  observations. 

The  axial  plane  of  a  fold  intersects  the  crest  of  trough  in  such  a 
manner  that  the  limbs  or  sides  of  the  fold  are  more  or  less  symmet- 
rically arranged  with  reference  to  it.  The  intersection  of  the 
axial  plane  with  the  crest  or  trough  of  a  fold  is  the  axial  line,  axis, 

104 


ELEMENTS   OF   FOLDS  105 

crest  line,  or  trough  line.  The  pitch  of  the  fold  is  the  inclination  of 
the  axial  line  to  the  horizontal.  It  is  merely  a  special  case  of  dip 
taken  along  the  axis. 

Strike  and  pitch  are  never  strictly  parallel,  although  if  the  pitch 
is  slight,  they  may  be  nearly  so. 

A  simple  fold  is  a  single  bend  or  curve  without  minor  crenula- 
tions.  A  composite  fold  is  the  simple  fold  with  minor  crenulations 
superposed  on  it.  A  complex  fold  is  one  which  is  cross  folded, 
that  is,  one  of  which  the  axial  line  is  folded.  As  denned  by  Van 
Hise,  composite  refers  to  two  dimensions,  or  the  cross  section,  and 
complex  to  three  dimensions.1 

As  practically  all  rock  folds  are  complex,  it  appears  that  the 
terms  " simple"  and  " composite"  merely  apply  to  descriptions 
of  cross  sections  of  complex  folds.  It  is  not  always  easy  in  dis- 
cussing folds  to  discriminate  clearly  between  a  consideration  of 
two  dimensions  and  of  three  dimensions,  and  hence  the  use  of  the 
terms  " composite"  and  " complex"  is  in  practice  frequently  loose. 
The  terms  are  useful,  however,  in  keeping  clearly  before  us  the 
desirability  of  discrimination  between  two-dimension  and  three- 
dimension  treatment  of  folds. 

The  axes  of  minor  folds  may  have  almost  any  angle  with  refer- 
ence to  the  axis  of  the  major  fold,  but  there  is  a  marked  tendency 
to  have  a  similar  angle  of  pitch  and  a  constant,  though  small, 
difference  in  strike. 

Anticline  and  syncline  refer  respectively  to  the  arch  and  trough 
of  a  simple  fold.  Anticlinorium  and  synclinorium  refer  to  com- 
posite arches  and  troughs.  Some  of  the  great  simple  flexures  of  the 
earth  have  been  called  by  Dana  geanticlines  and  geosynclines.'2 

Each  of  these  kinds  of  folds  may  be  further  classed  as  upright, 
inclined,  overturned,  or  recumbent,  depending  upon  whether  its 
axial  plane  is  vertical,  inclined,  overturned,  or  recumbent.  No 
further  definitions  of  these  terms  seem  necessary.  Where  the 
limbs  of  a  fold  are  parallel,  it  is  called  isoclinal.  When  the  axial 
planes  of  the  minor  folds  of  an  anticlinorium  converge  downward, 
the  fold  is  called  by  Van  Hise  a  normal  anticlinorium;  a  fan  fold 
is  a  special  case  of  this  (Fig.  49).  If  they  converge  upward  it  is 

1  Van  Hise,  C.  R.,  Principles  of  North  American  pre-Cambrian  geology:  16th 
Ann.  Kept.,  U.  S.  G.  S.,  part  1,  1896,  p.  603  et  seq. 

2  Dana,  James  D.,  Manual  of  geology,  4th  ed.,  1895,  p.  106. 


106 


STRUCTURAL   GEOLOGY 


called  an  abnormal  anticlinorium;  roof  structure  is  a  special  case  of 
this  (Fig.  50).    A  similar  division  applies  to  synclinoria. 

Minor  folds  are  commonly  developed  in  weak  beds  by  the 
shearing  between  two  more  competent  masses  of  rock.     These 


FIG.  49.  Generalized  fan  fold  or  normal  anticlinorium  of  central  massif  of  the  Alps. 

After  Heim. 

folds  are  conveniently  designated  drag  folds.  The  position  of  their 
axial  planes  is  controlled  by  the  displacement  of  the  more  com- 
petent beds  adjacent.  The  term  "drag  fold"  is  desirable  as 
emphasizing  the  differential  movement  between  the  controlling 
beds. 


FIG.  50.  Generalized  section  of  roof  structure  or  abnormal  anticlinorium  of  the 
central  massif  of  the  Alps.    After  Heim. 

A  parallel  fold  (Fig.  51)  is  one  in  which  there  is  no  thickening  or 
thinning  of  the  beds;  the  bedding  surfaces  are  mutually  parallel. 
The  curvature  of  no  two  beds  is  exactly  the  same.  This  difference 
in  curvature  implies  the  dying  out  of  folds  in  .one  direction  or 
another  from  a  given  bed,  and  the  differential  slipping  between  the 
layers  to  allow  for  the  dying  out  and  differing  curvature. 

In  similar  folds  (Fig.  51)  the  beds  are  thickened  and  thinned,  the 
bedding  surfaces  are  not  mutually  parallel,  but  the  curvature  is 
the  same  for  all  beds.  This  does  not  require  the  dying  out  of  folds 
or  differential  movement  between  beds 


ELEMENTS   OF   FOLDS 


107 


FIG.  51.  Figures  illustrating  (a)  ideal  parallel  and  (b)  ideal  similar  folds. 

Van  Hise. 


After 


108  STRUCTURAL   GEOLOGY 

FOLDS  IN  THE  ZONE  OF  FRACTURE  AND  ZONE  OF  FLOW  CON- 
TRASTED 

Rocks  are  folded  by  fracture  or  flow  or  by  any  combination 
of  these  two  processes.  Folds  therefore  appear  in  either  the  zone 
of  fracture  or  the  zone  of  flow  or  in  the  zone  of  combined  fracture 
and  flow.  Folding  by  fracture  differs  in  certain  essential  charac- 
teristics from  that  by  flowage. 


FIG.  52.  Folding  of  brittle  and  soft  layers  contrasted  in  jasper.     The  broken  dark 
layers  are  chert,  the  light  layers  are  secondary  iron  oxide. 

Folds  may  be  formed  by  means  of  minute  displacements  along- 
numerous  joints  and  faults.  Folds  in  brittle  quartzite  beds  are 
commonly  of  this  type. 

There  is  no  interior  deformation  of  the  fauit  and  joint  blocks, 
and  there  is  no  thickening  or  thinning  of  the  beds  as  a  whole.  The 
top  and  bottom  of  a  bed  are  parallel  throughout.  The  fold  is  of 
the  " parallel"  type.  The  curvature  of  the  beds  so  folded  is  not 


FOLDS  OF  FRACTURE  AND  FLOW  CONTRASTED  109 

the  same  through  any  considerable  vertical  distance.  A  much 
folded  bed  may  be  replaced  above  or  below,  usually  below,  by  a 
much  less  folded  bed  or  one  which  is  deformed  almost  none  at  all; 
in  other  words,  there  is  a  dying  out  of  the  fold.  Disappearance  of 
folds  with  depth  is  discussed  on  pages  124-127.  The  difference  in 
the  shortening  of  the  adjacent  strata  involves  slipping  between  the 
beds.  This  slipping  is  really  of  the  nature  of  faulting,  although  the 


FIG.  53.  Folding  of  brittle  and  soft  layers  contrasted  in  jasper.     Note  the  tension 
cracks  in  the  brittle  layers. 

movements  are  not  ordinarily  described  as  faults,  on  account  of 
taking  place  parallel  to  the  bedding. 

In  the  zone  of  fracture  rocks  are  relatively  competent;  they 
do  not  crumple  by  interior  adjustment;  the  folds  therefore  tend  to 
be  simple  and  open. 

In  the  zone  of  flowage  rocks  are  folded  by  interior  adjustment  of 
all  parts  of  the  mass  with  development  of  cleavage.  Beds  are 
thickened  and  thinned.  No  part  of  the  rock  mass  is  competent  to 
withstand  the  load  without  interior  adjustment  and  crumpling. 
The  result  is  a  much  more  composite  or  complex  folding.  The 
bed  thereby  becomes  thickened  and  strengthened,  enabling  it  to 


110 


FOLDS  OF  FRACTURE  AND  FLOW  CONTRASTED  111 

support  the  load.  The  folding  of  rocks  in  schistose  areas,  that  is, 
areas  which  indicate  that  they  have  been  in  the  zone  of  flowage,  is 
intricate  and  close,  and  contrasts  strongly  with  the  more  open  and 
simple  folding  of  rocks  in  the  zone  of  fracture.  For  instance,  the 
folding  in  the  Piedmont  area  of  Virginia,  the  rocks  of  which  were 
deformed  in  the  zone  of  flowage,  is  much  more  minute  and  com- 
plex than  that  of  the  Knox  dolomite  in  the  Appalachians  to  the 
west,  which  occurred  partly  in  the  zone  of  rock  fracture.  In  the 
folds  of  the  zone  of  flowage  the  readjustment  takes  place  not 
only  between  the  beds  but  in  every  part  of  the  bed.  The  curva- 
ture in  each  bed  tends  to  remain  the  same  as  in  the  strata  above 
and  below.  This  is  called  the  " similar"  type  of  folding.  (See 
Figs.  51b  and  54.)  The  distortion  in  the  layers  in  ideal  similar 
folds  is  greater  in  proportion  as  the  bends  are  gentle  on  the  anti- 
clines and  synclines.  Hence,  to  avoid  this  distortion,  there  is  a 
tendency  for  very  sharp  turns  at  these  places.  That  this  is  a 
controlling  tendency  may  be  observed  in  any  closely  plicated  area. 
The  actual  folds  of  a  closely  folded  mass  are  often  like  those  illus- 
trated in  Fig.  55. 

The  folds  of  the  zones  of  fracture  and  flow  therefore  contrast  in 
the  following  particulars : 

Zone  of  Fracture  Zone  of  Flow 

Beds  of  uniform  thickness.  Beds  thickened  and  thinned. 

No  interior  deformation.  Interior  deformation  of  all  parts. 

Relative  competence.  Relative  incompetence. 

Simple  outlines  of  competent  struc-  Crenulated  and  complex  outlines  of 

ture.  incompetent  structure. 

Much  slipping  between  beds;  dying  Little  slipping  between  beds;  per- 

out  of  folds  vertically.  sistence  of  folds  vertically. 

Folds  of  above  characteristics  are  Folds  of  above  characteristics  are 

"parallel."  "  similar." 

The  use  of  the  terms  competent  and  incompetent  respectively 
for  the  folds  of  the  zones  of  fracture  and  flow  require  some  further 
explanation.  Willis'  experiments  on  the  mechanics  of  Appala- 
chian structure1  showed  that  the  thicker,  more  competent  wax 
layers  rise  in  simple  outline  under  given  conditions  of  pressure  and 
load  until  they  are  unable  to  lift  the  load  farther.  Then  they 

1  Willis,  Bailey,  Mechanics  of  Appalachian  structure:  13th  Ann.  Kept.  U.  S. 
Geol.  Survey,  Pt.  2,  1893,  pp.  241-253. 


112 


STRUCTURAL   GEOLOGY 


crumple  and,  in  crumpling,  thicken,  enabling  them  to  lift  the 
load  higher.  Thus  composite  folds  are  really  indications  of  incom- 
petence. Simple  folds  are  more  characteristic  of  the  zone  of 
fracture;  the  bed  is  able  to  lift  itself  without  interior  readjustment, 


FIG.  55.  Folded  schist  from  Alaska.    Folds  are  "similar"  but  the  sharpness  of  the 
bends  involves  a  minimum  of  distortion  of  the  beds. 

and  without  crumpling;  it  is  competent.  All  folds  represent  a 
yielding  to  pressure.  In  that  sense  all  are  incompetent,  and  it 
might  be  better  to  speak  of  them  all  in  terms  of  degrees  of  incom- 
petency.  There  is  likely,  however,  to  be  little  confusion  in  follow- 
ing Willis  in  the  use  of  the  two  terms  competent  and  incompetent.1 

1  Op.  cit.,  p.  250. 


FOLDS  OF  FRACTURE  AND  FLOW  CONTRASTED  113 

Our  field  of  observation  is  practically  confined  to  the  zone  of 
combined  fracture  and  flowage,  and  hence  to  folds  representing 
some  combination  of  the  characteristics  of  the  two  zones.  The 
folds  described  as  typical  of  these  zones  may  be  regarded  as  the 
limiting  cases  between  which  all  folds  may  be  classified.  To 
illustrate,  interlayered  quartzite  and  slate  beds  exhibit  folds 
characteristic  of  both  zones.  The  quartzite  folds  may  be  of  the 
zone  of  fracture,  the  shale  folds  may  be  of  the  zone  of  flow.  The 
quartzite  layers  are  in  simple,  broad,  competent  folds  of  the 
" parallel"  type  developed  by  fracture  without  thickening  or 
thinning;  the  intervening  slate  layers  are  crenulated,  thickened, 
and  thinned,  relatively  incompetent,  and  of  the  "similar"  type. 
There  are  many  folds  in  homogeneous  beds  with  characteristics 
intermediate  between  those  described  for  fracture  and  flowage 
conditions. 

The  principal  use  of  such  a  classification  is  not  alone  to  afford  a 
means  of  pigeon-holing  various  folds,  but  to  call  attention  to  the 
characteristics  of  folds  which  it  is  desirable  to  know  for  field  study. 
The  attempt  to  analyze  a  fold  in  the  field  and  determine  what 
combination  of  fracture  and  flowage  conditions  it  represents  will 
lead  to  a  better  understanding  of  the  structure  than  will  the  mere 
naming  of  the  fold  according  to  form.  For  instance,  explorations 
for  iron  ore  have  been  going  on  extensively  in  a  great  slate  area, 
completely  covered  by  glacial  drift,  in  central  Minnesota.  Drilling 
soon  demonstrated  the  fact  that  the  slate  was  folded  in  the  zone  of 
flowage.  The  observer  was  therefore  justified  in  concluding  that 
the  folding  was  probably  close  and  complex,  that  there  was  much 
thickening  and  thinning  of  the  beds,  that  the  folds  were  largely  of  a 
similar  type,  not  dying  out  above  or  below.  The  application  of 
these  principles,  therefore,  has  been  of  great  aid  in  interpreting 
fragmentary  records  brought  up  from  the  drill  holes,  has  made  it 
possible,  for  instance,  to  correlate  a  thirty-foot  bed  of  ore  on  the 
limb  of  a  fold  with  a  fifty-foot  bed  near  the  crest.  In  the  Mar- 
quette  district  of  Michigan,  where  there  are  beds  of  quartzite 
interbedded  with  softer  slates  and  iron  formation,  it  has  been 
possible  by  the  application  of  these  principles  to  correlate  some  of 
the  simpler  and  broader  structures  of  the  quartzites  with  the  closer, 
much  more  complex,  and  quite  different  folds  of  the  softer  beds. 
In  making  any  satisfactory  estimate  of  the  thickness  of  folded  beds 


114  STRUCTURAL   GEOLOGY 

the  first  question  to  be  settled  is  the  degree  in  which  the  folds  are 
characteristically  those  of  the  zone  of  flowage  and  therefore  to  what 
extent  they  are  likely  to  be  thickened  or  thinned. 

CONTROL  OF  STRUCTURES  IN  WEAK  BEDS  BY  DIFFERENTIAL 
MOVEMENTS  BETWEEN  COMPETENT  BEDS  ON  LIMBS  OF 
FOLDS 

Rocks  within  our  field  of  observation  are  of  varied  competence. 
It  follows  then  that  in  any  folded  area  the  structures  of  the  weaker 
rocks  are  controlled  by  the  folding  of  the  stronger  beds.  The 
stronger  beds  tend  to  assume  the  " parallel"  type  of  folds  in  which 
the  principal  readjustment  is  between  the  beds  rather  than  within 
them.  This  readjustment  or  slipping  is  concentrated  in  the  inter- 
vening weaker  layers.  The  structures  of  the  weaker  layers  indicate 
the  direction  of  this  readjustment  and  thus  something  of  the  struc- 
ture of  the  competent  beds.  This  fact  is  of  great  aid  in  the  field 
study  and  interpretation  of  a  folded  area. 

Differential  movement  between  beds  is  uniformly  toward  con- 
vex surfaces  in  the  manner  indicated  in  the  diagram  (Fig.  56). 
In  the  following  pages  several  criteria  will  be  mentioned  by  which 
the  direction  of  differential  movement  may  be  determined  in 
the  field.  Knowing  the  direction  of  such  movement,  it  is  possible 
to  relate  the  minor  structures  to  the  major  folds. 

(1)  Minor  Folds  as  Evidence  of  Differential  Movement  Between 
Beds. — When  areas  of  heterogeneous  rocks  are  folded  the  stronger, 
more  competent  layers  are  likely  to  show  the  characteristics  of  folds 
of  the  zone  of  fracture,  and  the  softer,  more  incompetent  layers  to 
show  the  characteristic  folds  of  the  zone  of  flow,  although  the  two 
kinds  of  folds  may  represent  neither  one  extreme  nor  the  other. 
The  folds  of  the  weaker  layers  are  really  "drag  folds"  due  to 
differential  movement  between  the  controlling  harder  layers. 
The  inclination  of  the  axial  planes  of  the  minor  folds  with  reference 
to  the  adjacent  competent  beds  tells  the  direction  of  the  differen- 
tial movement.  The  axial  planes  are  nearly  parallel  to  cleavage 
(see  pp.  119-120).  When  this  movement  is  of  great  proportions 
the  axial  planes  of  the  minor  folds  may  become  so  rotated  as  to 
give  the  abnormal  type  of  composite  fold.  Beds  of  shale  may 
indicate  a  differential  movement  of  quartzite  beds  above  and  be- 
low in  the  direction  shown  on  diagram  56. 


DRAG   FOLDS 


115 


The  position  of  the  major  fold  is  inferred  from  the  differential 
movement  indicated  by  the  minor  folds.  The  major  fold  may  in 
turn  be  found  to  be  one  of  a  series  of  minor  folds  related  to  a  still 
larger  fold. 

This  is  something  more  than  the  statement  of  an  academic 
principle.  The  writer  regards  it  as  one  of  the  most  fundamental 
principles  in  the  field  study  of  structures.  Adherence  to  the 
simple  plan  of  watching  for  indications  of  differential  movement 
leads  to  surprising  results.  In  the  Lake  Superior  pre-Cambrian 


FIG.  56.  Figure  showing  differential  movement  between  competent  beds  on  limbs 
of  a  fold  with  the  development  of  minor  drag  folds  between  them. 

districts  it  has  been  possible,  by  studying  the  minute  crenula- 
tions  of  the  softer  beds,  to  determine  the  differential  movement 
of  the  controlling  strata  on  each  side,  and  thereby  to  obtain  a 
notion  of  the  position  of  the  next  larger  unit  of  structure.  This 
has  led  to  a  study  of  still  larger  units,  and  so  on.  In  the  Mar- 
quette  district  of  Michigan  the  slate  beds  are  folded  in  the  manner 
to  be  expected  from  the  control  of  the  harder  quartzite  layers  of 
the  Marquette  synclinorium.  Understanding  this  relation,  the 
composite  outlines  of  the  slate  folds  may  be  satisfactorily  corre- 
lated with  the  simple  outlines  of  the  quartzite  folds.  The  Mar- 
quette synclinorium  as  a  whole  may  be  regarded  as  a  minor  fold 
showing  differential  movement  upon  the  limb  of  the  major  Lake 
Superior  synclinorium. 


116 


STRUCTURAL   GEOLOGY 


FIG.  57.  Photograph  of  drag  fold  in  sedimentary  beds.    After  Hotchkiss. 

The  principle  of  the  control  of  minor  by  major  folds  affords  the 
most  reasonable  hope  of  working  out  successfully  the  complex 
structures  of  the  old  Archean  or  Basement  Complex,  which  hereto- 
fore have  been  regarded  as  almost  inexplicable.  Although  on 
casual  inspection  the  folds  in  any  ledge  show  an  apparently  great 


DRAG   FOLDS  117 

complexity,  when  examined  with  reference  to  the  differential 
movement  the  general  structure  becomes  more  manifest  and  it  is 
possible  to  infer  some  of  the  relations  of  the  major  folding.  By  the 
use  of  this  principle  it  has  been  possible  recently  to  work  out  the 
structure  of  certain  parts  of  the  closely  folded  Archean  of  the 
Vermilion  district  of  Minnesota,  which  have  heretofore  been  des- 
ignated simply  as  Basement  Complex.  The  writer  has  observed, 
in  traveling  over  hundreds  of  miles  of  Laurentian  gneiss,  that 
minor  folds  are  accordant  over  considerable  areas,  indicating  some 
major  control  and  suggesting  the  possibility  of  working  out  larger 
units  of  structure. 

The  "decken"  structure  of  the  Alps,  illustrated  by  Fig.  58, 
is  a  series  of  great  overthrust  folds  with  nearly  parallel  and  hori- 
zontal axial  planes,  which  are  probably  to  be  regarded  on  a  large 
scale  as  minor  "drag  folds  "  resulting  from  the  horizontal  shearing 
of  some  formerly  existing  competent  rocks  over  the  Alpine  area. 
The  great  Alpine  fan  folds  of  the  type  so  well  known  through  the 
writings  and  sections  of  Heim1  and  others  are  now  being  largely 
interpreted  by  Schardt,  Lugeon,2  and  others  as  "decken  "  or  over- 
thrust  folds  and  faults.  The  actually  observed  structures  seem  to 
permit  of  connections  in  cross  sections  drawn  to  correspond  to 
either  hypothesis,  and  it  is  probably  uncertain  in  some  cases  which 
interpretation  is  the  correct  one. 

As  folds  usually  have  a  pitch,  the  axial  lines  of  minor  drag 
folds  when  projected  to  the  surface  uniformly  vary  a  few  degrees  in 
strike  from  the  strike  of  the  beds  at  the  surface,  in  all  cases  where 
the  axial  lines  are  not  horizontal  nor  the  dips  vertical.  This  is  well 
illustrated  by  folds  in  iron  formation  of  the  Menominee  district  of 
Michigan  (Fig.  59).  At  one  place  the  iron  formation  dips  70° 
N.  and  strikes  N.  70°  W.  The  pitch  of  the  minor  folds  is  30°  in  a 
direction  N.  65°  W.  As  the  pitch  carries  these  folds  down  they  are 
carried  northward  down  the  dip  of  the  beds.  Hence  there  is  a 
divergence  of  5C  in  this  case  between  the  surface  projection  of  the 
axial  line  of  the  minor  fold  and  the  strike  of  the  bedding,  which  is  a 
fact  of  some  commercial  significance  in  the  exploration  for  ore,  in 
view  of  the  fact  that  the  ore  follows  the  pitch  rather  than  the  strike. 

1  Heim,  Alb.,  Untersuchunger  liber  den  Mechanismus  der  Gebirgsbildung,  Basel, 
1878. 

2  See:  Der  Bau  der  Schweizerlapen,  by  Alb.  Heim:  Neujahrsblatt  der  Natur- 
forschenden  Gesellschaft  in  Ziirieh  auf  das  Jahr  1908,  110  Stuck. 


118 


STRUCTURAL   GEOLOGY 


FIG.  58.  To  illustrate  development  of  overthrust  folding  and  faulting,  accom- 
panied by  minor  drag  folds,  as  inferred  from  Alpine  structure.    After  Heim. 


DRAG   FOLDS 


119 


The  application  of  this  principle  of  differential  movement  not 
only  indicates  the  position  of  the  minor  fold  with  reference  to  the 
major  fold  but  sometimes  affords  a  means  of  determining  which  is 
the  top  and  which  the  bottom  of  a  bed  on  the  limb  of  a  fold.  If 
in  an  isolated  outcrop  of  vertical  beds  it  is  apparent  that  the  left 
hand  side  has  moved  up  with  reference  to  the  right  hand  side,  the 
inference  is  that  the  ledge  is  a  part  of  the  left  limb  of  an  anticline. 
If  so,  the  left  hand  beds  of  the  outcrop  constitute  the  top.  This 
method  has  application  in  the  study  of  isolated  outcrops  in  which 
no  other  evidence  of  top  or  bottom  appears. 


FIG.  59.  To  illustrate  divergence  in  strike  and   pitch.     After  Mead. 

(2)  Cleavage  as  Evidence  of  Differential  Movement  in  Folding. — 
Fracture  cleavage  or  flow  cleavage  is  usually  associated  with 
the  weaker  beds  in  folds.  The  attitude  of  cleavage  with  reference 
to  the  bedding  indicates  the  direction  of  differential  movement 
between  the  beds,  and,  like  the  drag  fold,  becomes  an  aid  in  inter- 
preting structure  (see  Figs.  9-11,  37,  46,  47).  When  a  slate  or  shale 
is  folded  between  two  competent  layers,  such  as  quartzite,  the 
cleavage  produced  in  the  slate  affords  clear  evidence  of  slipping  or 
shearing  between  the  quartzite  beds.  The  cleavage  is  inclined  to 
the  bedding  at  angles  determined  by  the  amount  of  slipping,  and 
tends  to  converge  upward  on  an  anticline  of  gentle  curvature. 


120  STRUCTURAL   GEOLOGY 

The  cleavage  is  approximately  parallel  to  the  axial  planes  of  minor 
drag  folds  which  are  likely  to  be  present  under  such  conditions 
(see  p.  114). 

Some  beds  are  so  closely  compressed  that  both  the  cleavage 
and  the  bedding  are  so  nearly  vertical  as  to  be  about  parallel. 
Here  it  may  be  impossible  to  apply  the  above  simple  principles 
of  relationship;  but  the  writer  has  found  that  when  a  detailed 
study  has  been  made  it  has  sometimes  been  possible  even  here  to 
draw  some  reasonable  inference  as  to  the  position  of  the  axial 
planes  of  the  closely  compressed  folds.  In  irregular  anticlinoria 
or  synclinoria  the  cleavage  may  apparently  have  such  intricate 
relations  to  bedding  that  it  is  impossible  to  formulate  any  general 
statement  of  the  relations  of  cleavage  to  the  fold  as  a  whole. 
Examination  of  any  detail  of  the  fold,  however,  will  indicate  the 
relations  above  described,  and  from  these  details  much  light  may 
be  thrown  upon  the  general  character  of  the  fold. 

When  all  parts  of  a  homogeneous  incompetent  rock  are  folded  in 
the  zone  of  rock  flowage,  there  is  a  less  pronounced  shearing  be- 
tween beds,  and  less  control  of  cleavage  directions  by  differential 
movements  on  the  limbs.  The  cleavage  may  have  a  uniform  dip 
regardless  of  folds,  but  in  general  is  parallel  to  the  axial  planes. 
Both  the  cleavage  and  the  folds  may  then  be  regarded  as  having 
been  developed  under  some  larger  control,  as,  for  instance,  the 
shearing  of  a  rigid  mass  horizontally  over  the  entire  area.  This  is 
illustrated  by  monoclinal  cleavage  crossing  the  complex  folds  in 
slate  without  any  apparent  relation  to  the  minor  folds;  whereas 
when  compared  with  major  folds  in  adjacent  competent  strata  the 
cleavage  is  found  to  be  in  positions  which  indicate  its  development 
under  the  control  of  a  major  fold.  The  same  principle  on  a  larger 
scale  may  be  considered  as  explaining  some  of  the  regional  cleavage. 

Large  areas  like  the  Piedmont  Plateau  and  parts  of  the  pre- 
Cambrian  shield  of  North  America  have  a  cleavage  with  remark- 
ably uniform  strike  and  dip,  notwithstanding  the  heterogeneity  of 
rocks  and  folds  on  these  areas.  There  seems  to  have  been  some  one 
factor  controlling  the  development  of  cleavage  for  the  area  as  a 
whole.  There  is  no  reason  to  believe  that  the  development  of  such 
cleavage  does  not  conform  to  the  laws  of  stress  and  strain  already 
described,  but  the  units  of  structure  involved  may  be  much 
larger  than  those  observable  in  the  individual  folds. 


CLEAVAGE   AND   FOLDS  121 

The  fact  that  the  cleavage  is  often  inclined  rather  than  vertical 
suggests  horizontal  shearing  stresses  rather  than  horizontal  non- 
rotational  compression  (see  pp.  16-21);  and  these  might  be  de- 
veloped by  the  shearing  of  a  large  portion  of  the  zone  of  fracture 
over  an  underlying  zone  of  flow.  Chamberlin1  has  suggested  that 
certain  great  thrust  planes  may  be  the  surface  expression  of  a 
deep  zone  of  shearing.  Shearing  movement  has  been  thought  by 
Chamberlin2  to  be  a  possible  result  of  creep,  under  gravity,  of  the 
elevated  portions  of  the  earth's  crust,  especially  at  continental 
margins.  Van  Hise3  has  suggested  that  perhaps  tidal  friction, 
tending  to  retard  the  surface  of  the  earth  in  its  rotation,  might 
give  it  a  tendency  to  shear  relatively  westward  over  underlying 
portions,  thereby  giving  eastward-dipping  cleavage.  Strikes  and 
dips  of  cleavage  have  not  been  sufficiently  well  correlated  over 
large  areas  to  ascertain  to  what  extent  they  might  correspond 
with  the  requirements  of  any  one  of  these  hypotheses. 

(3)  Jointing,  Fracture-Cleavage,  and  Fissility  as  Evidences  of 
Differential  Movement  Between  Beds  in  Folding. — Differential 
movement  between  beds  develops  one  set  of  shearing  planes  parallel 
to  the  beds  and  another  at  an  angle  less  than  90°  to  it  (see  Fig.  7) . 
The  latter  set  indicates  the  direction  of  the  displacement  between 
beds  in  folding.  Joints  or  fracture  cleavage  form  along  these 
planes.  They  may  be  curved  or  S-shape.  Also  they  are  likely  to 
be  confined  to  certain  beds  and  offset  along  the  bedding  plane 
in  passing  to  different  strata  on  either  side.  Given,  then,  joints 
thus  obviously  related  to  folding,  it  is  possible  to  determine  the 
differential  movement  and  get  a  notion  as  to  the  part  of  the  fold 
on  which  observation  is  taken.  For  instance,  in  the  Baraboo  dis- 
trict of  Wisconsin,  northward  dipping  beds  of  quartzite  are  cut 
by  two  sets  of  joints,  one  set  parallel  to  the  bedding,  and  another 
set  of  strike  joints  crossing  the  bedding  and  dipping  northward 
(see  Figs.  10  and  11).  It  is  clear  in  this  instance  that  the  upper  beds 
have  moved  southward  with  reference  to  the  lower  beds.  This  cor- 
responds to  the  requirements  of  a  position  on  the  south  limb  of  a 
syncline.  The  same  kind  of  reasoning  may  be  applied  to  the  north 
limb  of  the  Baraboo  syncline  (see  Fig.  9). 

1  Chamberlin,  T.  C.,  The  fault  problem:  Econ.  Geol.,  Vol.  2,  1907,  p.  598. 

2  Idem.,  p.  718. 

3  Van  Hise,  C.  R.,  A  treatise  on  metamorphism:  Mon.  47,  U.  S.  Geol.  Survey, 
1904,  p.  930. 


122 


STRUCTURAL   GEOLOGY 


FIG.  60.  Illustrating  the  artificial  development  of  fold.  After  Willis.  The  fold 
begins  to  develop  at  points  of  initial  irregularity  in  the  beds  (initial  dip)  near 
the  point  of  application  of  force.  The  heavy  layers  rise  in  simple,  com- 
petent, parallel  folds,  the  soft  layers  in  composite,  incompetent,  similar  folds. 
When  the  stronger  layers  have  risen  to  the  limit  of  their  competency  they 
buckle,  developing  composite  outlines  and  to  that  extent  taking  on  character- 
istics of  incompetent  folds. 


It  is  frequently  necessary  to  interpret  structure  from  a  few 
widely  separated  exposures,  and  then  relations  of  this  type  may 
furnish  a  clue  to  the  structure,  to  be  checked  of  course  by  other 
criteria. 


DIFFERENTIAL   MOVEMENTS   IN   FOLDS        123 


Fig.  60  (continued.) 

Conclusion  as  to  differential  movements  in  deformation.  Our  field 
observation  being  confined  to  the  zone  of  combined  fracture  and 
flowage  where  the  beds  are  both  competent  and  incompetent, 
it  follows  that  the  larger  part  of  the  rock  structures  and  the 
rock  deformation  described  in  this  book  may  be  regarded  as 
evidences  and  results  of  differential  movement  on  a  smaller  or 
larger  scale. 


124  STRUCTURAL   GEOLOGY 

LOCALIZATION   OF   FOLDS 

(a)  It  has  been  shown  experimentally  by  Willis1  that  folds  tend 
to  form  near  the  point  of  application  of  the  deforming  force,  unless 
the  rocks  are  sufficiently  rigid  to  transmit  the  thrust  forward  to 
some  weaker  zone,  (b)  Willis  has  also  shown  that  slight  irregular- 
ity in  the  bedding,  such  as  might  be  formed  during  sedimenta- 
tion, and  which  he  calls  initial  dip,  will  tend  to  localize  a  fold,  even 
at  some  distance  from  the  point  of  application  of  the  stresses. 
(c)  Still  further,  it  appears  that  the  uplift  of  the  fold  at  any  one 
place  may  tend  to  depress  the  beds  immediately  beyond  it,  creating 
an  irregularity  or  "  initial  dip  "  which  localizes  another  fold.  The 
first  fold  rises  to  such  a  point  that  it  becomes  easier  to  develop  a 
new  fold  than  to  lift  the  old  fold  higher,  (d)  Irregularities  of 
structures  other  than  bedding  may  localize  the  fold.  Contact  of 
rocks  of  unequal  strength,  for  instance  of  granite  and  sediments, 
has  been  observed  to  localize  folds,  the  massive  granite  serving  as  a 
buttress  against  which  the  weaker  series  is  deformed,  (e)  Inherent 
weakness  of  rocks  may  localize  a  fold.  A  slate  is  likely  to  be  more 
folded  than  an  adjacent  quartzite.  Initial  dip  or  other  irregularity 
may  determine  at  what  points  in  the  shale  the  folds  shall  be 
localized,  but  the  weakness  of  the  shale  as  a  whole  as  compared 
with  the  adjacent  beds  will  favor  the  development  of  folds  in  the 
shale  rather  than  in  the  quartzite.  This  weakness  is  one  of  the 
common  causes  for  the  localization  of  folds. 

DETERMINATION  OF  DEPTH  AFFECTED   BY  FOLDS 

It  is  sometimes  possible  to  measure  the  linear  shortening  of  an 
area  by  folding,  and  also  the  vertical  uplift.  These  are  necessary 
data  for  estimating  the  depth  affected  by  the  folding.  In  the 
following  diagram,  which  may  be  supposed  to  illustrate  roughly 
the  Southern  Appalachian  folding,  100  miles  of  surface  has  been 
crowded  into  75  miles.  There  has  been  an  uplift  of  approximately 
a  mile.  Obviously  the  product  of  the  linear  uplift  and  the  length 
of  the  shortened  area,  1  mile  x  75  miles,  should  equal  the  product 
of  the  shortening,  25  miles,  and  the  depth  affected.  By  solving  the 
equation,  this  depth  is  found  to  be  3  miles.  This  method  was 

1  Willis,  Bailey,  Mechanics  of  Appalachian  Structure:  13th  Ann.  Kept.  U.  S. 
Geol.  Survey,  pt.  2,  1893,  p.  247. 


DEPTH   OF   FOLDS 


125 


suggested  by  T.  C.  Chamberlin  1  and  has  been  applied  by  R.  T. 
Chamberlin2  to  the  Appalachian  folds  of  central  Pennsylvania. 
A  similar  method  was  independently  developed  by  Willis3  and 
applied  to  the  Cascade  Mountains.  The  same  method  has  been 
suggested  by  T.  C.  Chamberlin4  to  determine  depth  affected  by 
faults. 

With  a  given  elevation,  the  less  close  the  folding  (or  faulting) 
and  therefore  the  less  the  shortening,  the  greater  the  vertical 
distance  involved  in  the  deformation.  In  the  section  made  by 


« Prfsenf  /ertgth  of  folded  sect  /on          > 


Average  ujo/ift  — * 
* 


Deforming  farce 


FIG.  61.  Illustrating  a  method  of  determining  depth  affected  by  folds. 

R.  T.  Chamberlin5  from  Harrisburg  to  Tyrone  in  Pennsylvania 
he  finds  that  shallower  depths  are  affected  on  the  two  ends  of 
the  section  and  greater  depths  toward  the  center  (see  Fig.  62). 
The  shallowest  deformation  found  is  5.7  miles.  Making  calcula- 
tions for  five  sub-sections,  he  finds  a  gradual  increase  in  the  depth 
affected  toward  the  center  of  his  section,  which  suggests  that  the 
deformed  zone  is  bounded  by  planes  dipping  approximately  45° 
from  the  surface  at  either  end  of  the  section  and  intersecting  about 
32  miles  below  the  surface  near  the  center.  The  intersection  of 
these  hypothetical  planes  at  45°  with  each  other  and  with  the 
earth's  surface  suggests  to  Chamberlin  that  they  are  really  shearing 

1  Chamberlin,  T.  C.,  and  Salisbury,  R.  D.,  Geology,  Vol.  II,  1906,  pp.  125-126. 

2  Chamberlin,  R.  T.,  Appalachian  folds  of  central  Pennsylvania;  Jour.  Geol., 
Vol.  18,  1910,  pp.  228-251. 

3  Willis,  Bailey,  Physiography  and  deformation  of  the  Wenatchee-Chelan  dis- 
trict, Cascade  Range:  Prof.  Paper  No.  19,  U.  S.  Geol.  Survey,  1903,  pp.  95-97. 

4  Chamberlin,  T.  C.,  The  fault  problem:  Econ.  Geol.,  Vol.  II,  1907,  p.  596. 

5  Op.  cit.,  p.  245. 


126 


STRUCTURAL  GEOLOGY 


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DEPTH   OF   FOLDS  127 

planes  developed  by  tangential  shortening  in  the  manner  of  frac- 
ture planes  formed  in  a  block  under  pressure. 

The  above  inference  implies  that  the  pressure  has  been  applied 
with  equal  intensity  on  all  unit  areas  on  the  sides  of  the  deformed 
block;  it  implies  a  non-rotational  strain;  it  implies,  further,  that 
shearing  planes  find  expression  in  the  zone  of  rock  flowage.  While 
shearing  stresses  are  undoubtedly  present  in  this  zone  during 
deformation,  it  is  not  so  clear  that  they  would  find  expression  in 
definite  planes  bounding  the  deformed  region  or  that  such  planes 
would  have  the  position  assumed  for  them.  They  would  not  if  the 
strain  were  rotational,  developed  by  tangential  stresses.  The 
structure  consonant  with  such  deformation  in  the  zone  of  flowage 
is  a  vertical  cleavage,  as  is  implied  by  Willis'  conclusion  concerning 
the  Cascade  folding.1  The  depth  reached  by  the  deforming  move- 
ments of  the  Cascade  uplift  has  been  calculated  by  Willis  to  be 
from  375  to  1500  miles.  The  smaller  of  these  estimates  would  lead 
so  deep  into  the  zone  of  flowage  as  to  make  it  impossible  to  con- 
sider the  deformation  as  being  controlled  by  shear  zones.  Willis 
seems  to  have  considered  that  the  entire  mass  has  been  shortened 
by  flowage  down  to  these  depths,  resulting  in  vertical  uplift. 

The  methods  for  the  determination  of  depths  of  folding  worked 
out  by  the  Chamberlins  are  of  fundamental  significance,  and  are 
likely  to  yield  unexpected  results. 

Another  way  of  estimating  the  depth  affected  by  folding  is  to 
compare  the  deformation  in  stratigraphically  superposed  rocks  in  a 
given  locality.  Daly2  finds  in  south-central  British  Columbia  that 
the  pre-Cambrian  massives  are  much  less  folded  than  the  overlying 
Carboniferous  and  Triassic  rocks,  indicating  that  a  small  depth  of 
the  earth  shell  has  suffered  strong  folding  in  post-Cambrian  time. 

FIELD  OBSERVATIONS  ON  FOLDS 

Strike  and  Dip: — Strike  and  dip  records  are  ordinarily  of  value 
because  of  the  light  they  throw  on  the  folding  of  strata.  It  is 
essential  in  taking  the  readings  to  keep  this  in  mind  in  selecting 
points  at  which  to  take  the  observations.  Especially  it  is  desirable, 

1  Willis,  Bailey,  Physiography  and  deformation  of  the  Wenatchee-Chelan  dis- 
trict, Cascade  Range:  Prof.  Paper  No.  19,  U.  S.  Geol.  Survey,  1903,  pp.  92-97. 

2  Daly,  R.  A.,  Abstract  of  paper  presented  at  24th  annual  meeting  of  Geol.  Soc. 
Am.  at  Washington,  D.  C.,  December,  1911. 


128  STRUCTURAL   GEOLOGY 

as  soon  as  the  existence  of  a  fold  is  suspected,  to  search  for  the 
axis,  in  order  to  ascertain  the  pitch.  In  a  closely  folded  area  the 
deformation  of  the  beds  by  shearing  on  the  limbs  is  so  much  greater 
than  on  the  axial  lines  that  frequently  much  can  be  ascertained 
from  a  study  of  the  axial  zone  which  could  not  be  suspected  from  a 
study  of  the  limbs  alone.  An  illustration  may  be  cited  from  the 
folded  Algonkian  and  Archean  rocks  in  the  Vermilion  district  of 
Minnesota.  The  Archean  is  exposed  in  the  cores  of  closely  folded 
anticlines.  Along  the  sides  of  these  anticlines  the  shearing  is  so 
close  that  cleavage  has  been  developed  both  in  Archean  and  Algon- 
kian, and  the  evidence  of  their  relations  practically  destroyed. 
On  the  axis  of  the  fold,  however,  where  it  pitches  under  the  sur- 
face, it  is  frequently  possible  to  find  the  beds  so  little  deformed 
that  conglomerates  may  be  recognized  and  the  relations  worked 
out. 

The  determination  of  the  pitch  of  the  axis  gives  the  dip  of  the 
limb  of  the  cross  fold. 

The  taking  of  strike  and  dip  observations  at  random  without 
a  definite  attempt  to  correlate  them  on  to  the  general  structure 
of  the  district  at  the  time  they  are  taken  leads  frequently  to 
unsatisfactory  results.  Daily  field  study  of  strike  and  dip  observa- 
tions, conscientiously  platted  to  date,  should  be  the  basis  for 
planning  field  work  on  succeeding  days.  Too  frequently,  definite 
field  determinations  of  pitch  are  not  made,  but  are  left  to  be  in- 
ferred from  a  study  of  the  records  when  later  platted.  Thus 
one  of  the  most  important  and  decisive  elements  of  structure  is 
loosely  determined,  and  this  neglect  may  often  lead  to  serious  error. 

Emphasis  on  Relations  of  Major  and  Minor  Structures: — The  con- 
stant attempt  to  correlate  minor  and  major  structures  under  the 
principles  outlined  in  the  preceding  sections  cannot  be  too  strongly 
urged.  It  is  indeed  surprising  what  a  variety  of  applications  these 
principles  have.  It  is  seldom  that  a  study  of  any  element  of  the 
structure  does  not  give  a  clue  as  to  what  to  expect  in  the  larger  or 
smaller  elements  of  the  deformation. 

Field  Observations  on  Relations  of  Cleavage  to  Folds: — Keeping  in 
mind  the  simple  relationships  of  cleavage  to  folds,  discussed  on 
pages  119—121,  the  following  are  some  of  the  field  inferences  that 
may  be  drawn  from  cleavage.  The  student  will  find  it  to  his 
advantage  to  reason  out  each  of  these  inferences  for  himself. 


FOLDS   AND   CLEAVAGE 


129 


(a)  Cleavage  converging  upward  suggests  an  anticline.  It  is 
seldom,  however,  that  this  ideal  condition  may  be  recognized  in 
the  field  on  any  large  scale.  The  slight  overturning  of  cleavage  or 
folding  makes  it  difficult  to  determine  this  relation,  (b)  More 
useful  are  the  inferences  to  be  drawn  perhaps  from  local  observa- 
tions of  the  relation  of  cleavage  to  bedding.  Cleavage  normal  to 
bedding  probably  indicates  the  axial  plane  of  the  fold,  (c)  Cleav- 
age inclined  to  the  bedding  probably  indicates  the  limb  of  a  fold, 
(d)  The  inclination  of  the  cleavage  with  reference  to  the  bedding 
tells  on  which  limb  of  the  fold  the  observation  is  taken,  (e)  If 
bedding  is  vertical  and  inclined  cleavage  is  present  in  the  softer 


Shale  bed  -'^^^LJ^- 


'Second  /eye/ 


FIG.  63.  Vertical   section   of   Illinois  mine,   Baraboo   district,   Wisconsin.     After 

Weidman. 


layers  between  harder  ones,  thereby  indicating  direction  of  dis- 
placement, it  is  possible  to  infer  on  what  part  of  the  fold  this 
relation  was  doubtless  developed  and  from  this  in  turn  it  may  be 
inferred  which  is  the  top  and  which  the  bottom  of  the  bed. 

In  the  Baraboo  district  of  Wisconsin,  slate  overlain  by  iron 
formation  has  been  folded  between  a  competent  quartzite  layer 
below  and  a  dolomite  bed  above.  The  slate  thus  forms  the  foot- 
wall  for  the  iron  formation.  A  shaft  sunk  largely  in  the  slate 
followed  the  cleavage,  the  bedding  being  very  obscure.  As  a 
result,  the  shaft  penetrated  the  ground  more  steeply  than  the 
bedding,  as  would  be  expected,  and  where  at  considerable  depth 


130  STRUCTURAL   GEOLOGY 

drifting  was  begun  to  cross  cut  the  ore,  it  was  found  that  the 
bottom  of  the  shaft  was  a  long  distance  away  from  the  ore  body. 

The  greater  part  of  the  Lake  Superior  iron  ore  is  found  in  the 
upper  Huronian  group  of  rocks,  of  which  slate  is  an  important 
member.  Much  of  the  exploration  has  to  be  done  by  drilling, 
and  a  study  of  the  relations  of  cleavage  to  bedding  in  the  slate 
brought  up  in  the  drill  cores  frequently  gives  important  clues 
to  the  folding.  For  instance  (a)  a  vertical  drill  hole  discloses  a 
vertical  cleavage  with  a  horizontal  bedding.  The  inference  is 
that  the  hole  is  parallel  to  the  axial  plane  of  the  fold,  (b)  It  dis- 
closes cleavage  inclined  to  the  bedding.  The  inference  is  that  the 
limb  of  the  fold  has  been  penetrated,  (c)  A  hole  drilled  at  an  angle 
of  45°  to  the  horizon  brings  up  a  core,  the  longer  direction  of  which 
bisects  the  acute  angle  between  the  cleavage  and  bedding.  The 
general  trend  of  the  principal  elements  of  structure  of  the  district 
is  known.  It  is  not  known,  when  the  core  is  brought  up,  how  much 
it  has  been  rotated  in  the  hole,  and  thus  from  the  core  two  hy- 
potheses are  possible — that  the  bedding  is  nearly  horizontal  and 
the  cleavage  nearly  vertical,  or  that  the  bedding  is  vertical  and  the 
cleavage  horizontal.  The  fact  that  cleavage  in  these  slates  is 
usually  vertical  or  nearly  so  makes  it  necessary  in  the  majority  of 
cases  to  conclude  that  the  bedding  is  horizontal,  and  that  it  has 
been  cut  near  the  axis  of  a  fold. 

Many  other  specific  illustrations  might  be  given  to  show  the 
value  of  this  principle  in  field  work.  If  the  observer  of  drag  folds 
or  cleavage  will  in  every  case  ask  himself  what  is  the  displacement 
shown  by  these  structures  taken  in  detail  or  as  a  whole,  he  will  be 
able  to  determine  his  probable  position  with  reference  to  the 
next  larger  order  of  fold,  and  hence  to  direct  his  work  more  intelli- 
gently in  working  out  the  features  of  this  larger  element  of  struc- 
ture. 

If  cleavage  alone  is  observed,  with  unknown  relations  to  bed- 
ding, some  valuable  inferences  are  still  possible.  The  very  exist- 
ence of  cleavage  implies  failure  or  incompetence  on  the  part  of  the 
rock  and  this  in  most  cases  involves  folding.  The  writer  has  yet 
to  find  a  true  slate  which  does  not  have  some  folding  of  the  beds. 
It  may  be  inferred  also  that  the  folds  are  characteristic  of  flowage 
conditions,  that  is,  similar  folds  with  composite  outlines,  and  that 
their  axial  planes  are  nearly  parallel  to  the  cleavage.  This  incom- 


RIPPLE   MARKS 


131 


FIG.  64.  Photograph  of  (a)  ripple  marks  and  (b)  casts  of  ripple  marks. 
After  Van  Hise. 


132  STRUCTURAL   GEOLOGY 

petent  structure  is  almost  certainly  controlled  by  competent  struc- 
tures in  stronger  adjacent  rocks  wherever  they  may  be.  The  pre- 
vailing strike  and  dip  of  cleavage  may  suggest  where  and  what  the 
larger  competent  structure  is.  Cleavage  in  a  slate  area  may 
strike  east  and  west  and  dip  south  at  an  angle  of  45°.  The  inference 
is  that  here  are  similar  composite  folds  with  east-west  trend  and 
axial  planes  dipping  to  south;  further,  that  the  structure  was 
developed  by  the  relatively  northward  movement  of  some  over- 
lying competent  rocks  which  have  been  removed;  finally  this 
inferred  major  control  suggests  a  major  anticline  to  the  north. 
Determination  of  Top  and  Bottom  of  Sedimentary  Beds  in  a  Folded 
Area: — It  is  only  in  folded  beds  that  criteria  other  than  super- 
position are  necessary  to  determine  top  or  bottom  of  the  beds. 
When  the  folding  is  worked  out  the  problem  is  solved.  Any 
methods  used  for  determining  folds  therefore  apply  to  this  prob- 
lem. The  relations  of  cleavage,  joints  and  minor  drag  folds 
to  major  structure  discussed  above  therefore  help  to  determine 
which  is  top  and  which  is  bottom  of  beds.  There  are  primary 
structures  of  beds  which  may  also  be  used  to  advantage,  par- 
ticularly (a)  ripple  marks,  (b)  false  bedding  and  (c)  variations  in 
coarseness  of  grain. 

(a)  In  Fig.  64  the  normal  ripple  marks  and  their  casts  are  indi- 
cated.   It  will  be  noted  that  in  the  normal  ripple  marks  the  crests 
are  much  sharper  than  the  troughs,  and  that  the  troughs  may  have 
minor  crests  in  them.    When  the  beds  are  on  edge  or  overturned, 
these  facts  enable  one  to  tell  which  is  top  and  which  is  bottom. 

(b)  In  Fig.  65  it  will  be  noted  that  the  false  bedding  is  abruptly 
cut  off  by  overlying  beds  while  it  comes  in  contact  with  the  lower 
beds  by  a  tangential  curve.     If  the  outcrop  shown  in  the  photo- 
graph were  turned  on  edge  or  overturned,  there  would  still  be 
no  difficulty  in  determining  which  were  the  original  top  and  bottom 
of  the  beds. 

(c)  It  is  very  common  to  find  a  diminution  in  coarseness  of 
beds  from  the  bottom  toward  the  top.     Even  in  microscopic  sec- 
tions this  is  apparent.     The  beds  may  start  in  abruptly  with 
coarse  sediments,   these  gradually  become  finer-grained   above, 
and  the  next  bed  start  in  again  abruptly  with  coarser  sediments. 
There  is  little  difficulty  in  these  cases,  no  matter  what  the  folding, 
in  determining  the  original  top  and  bottom  of  the  beds.    This  has 


CROSS  BEDDING 


FIG.  65.  False  bedding  or  cross  bedding  in  sandstone.     Dalles  of  the  Wisconsin. 
After  Salisbury  and  Atwood. 


134  STRUCTURAL   GEOLOGY 

been  found  especially  useful  in  interpreting  drill  samples  from 
folded  rocks. 


SUGGESTIONS   FOR   LABORATORY  STUDY  OF   FOLDS 

The  following  questions  merely  suggest  a  desirable  line  of  laboratory 
study.  Teachers  will  multiply  illustrations. 

On  the  Cloud  Peak-Fort  McKinney,  Wyoming,  or  Oelrichs,  South 
Dakota-Nebraska,  folios  (Nos.  142  and  85,  U.  S.  Geol.  Survey),  how  can 
the  strike  of  the  beds  be  determined  from  the  geologic  map?  Show  also 
how  the  direction  and  approximate  angle  of  the  dip  can  be  found  from  the 
map.  On  the  Monterey,  Virginia- West  Virginia,  or  Ringgold,  Georgia- 
Tennessee,  folios  (Nos.  61  and  2,  U.  S.  Geol.  Survey),  determine  the  direc- 
tion and  degree  of  pitch  of  axial  lines  of  both  anticlines  and  synclines. 
Are  the  strike  and  pitch  parallel?  What  are  the  various  possible  relations 
between  them?  What  do  these  relations  signify?  Study  the  valleys  and 
outcrops  on  the  Sundance,  Wyoming-South  Dakota,  folio  (No.  127,  U.  S. 
Geol.  Survey)  using  the  geologic  map.  How  do  the  shapes  of  outcrops 
vary  with  different  relationships  between  the  dip  of  the  beds  and  the 
direction  and  gradient  of  the  valleys? 

On  the  Monterey,  Virginia- West  Virginia,  or  Three  Forks,  Montana, 
folios  (Nos.  61  and  24,  U.  S.  Geol.  Survey)  show  how  anticlines  may  be 
distinguished  from  synclines  by  the  study  of  outcrops  on  the  geologic 
map;  the  same  with  reference  to  anticlinoria  and  synclinoria  on  the  Mt. 
Mitchell,  North  Carolina,  and  Menominee,  Michigan,  maps  (folios 
Nos.  124  and  62,  U.  S.  Geol.  Survey). 

On  the  geologic  maps  of  the  Mt.  Mitchell,  North  Carolina,  folio  (No. 
124,  U.  S.  Geol.  Survey)  show  how  the  outcrops  themselves  indicate  that 
certain  folds  are  overturned;  that  some  of  the  folds  are  isoclinal. 

On  the  Maynardville,  Tennessee,  Bristol,  Virginia-Tennessee,  and 
Morristown,  Tennessee,  folios  (Nos.  75,  59,  and  27,  U.  S.  Geol.  Survey) 
study  the  relation  of  the  little  drag  folds  to  the  major  folds  of  the  region. 
Is  there  any  relation  between  the  drag  folds  and  certain  rock  formations? 
Why?  What  is  the  relation  between  the  pitch  of  the  drag  folds  and  that 
of  the  major  folds?  What  differential  movements  do  the  drag  folds  in- 
dicate and  where  and  of  what  type  are  the  major  folds? 

Examine  cross  sections  on  the  Maynardville,  Tennessee,  and  Morris- 
town,  Tennessee,  and  Mt.  Mitchell,  North  Carolina,  folios  (Nos.  75,  27, 
and  124,  U.  S.  Geol.  Survey)  and  the  Marquette,  Michigan,  monograph 
of  the  U.  S.  Geological  Survey  (Vol.  28).  Are  the  synclinoria  normal  or 
abnormal?  What  caused  the  one  type  to  be  developed  rather  than  the 
other? 

Are  the  folds  on  these  cross  sections  similar  or  parallel  or  are  both  types 
present? 

Which  of  the  folds  studied  on  foregoing  maps  were  formed  in  the  zone 
of  rock  flow  and  which  in  the  zone  of  fracture?  Why? 


LABORATORY  STUDY   OF   FOLDS  135 

Given  an  outcrop  of  steeply  inclined  beds,  what  are  the  various  phenom- 
ena to  be  looked  for  indicating  differential  movement  between  the  beds? 
(See  Figs.  9,  10,  11,  37,  46  and  47.) 

Having  determined  the  direction  of  the  differential  movement,  what 
inference  do  you  draw  as  to  type  of  folding,  as  to  location  and  pitch  of  the 
axes  of  the  major  folds,  as  to  the  top  and  bottom  of  the  beds? 

Study  the  charts  accompanying  Willis'  "  Mechanics  of  Appalachian 
Structure,"  *  with  a  view  to  answering  the  following  questions:  What  has 
determined  the  location  of  the  folds?  How  are  folds  repeated?  What 
determines  whether  the  fold  shall  be  simple  or  composite  in  outline? 
Which  of  the  folds  are  of  the  abnormal  type  and  why  have  these  developed? 
Are  the  folds  similar  or  parallel?  Are  they  characteristic  of  the  zone  of 
rock  fracture  or  the  zone  of  rock  flow? 

1  Willis,  Bailey,  Mechanics  of  Appalachian  Structure:  13th  Ann.  Kept.  U.  S.  Geol. 
Survey,  pt.  2,  1893,  pp.  211-281. 


MOUNTAINS 

TYPES   OF  MOUNTAINS 

Mountains  may  be  carved  by  erosion  from  undeformed  sedi- 
ments or  undeformed  igneous  rocks.  They  may  be  formed  en- 
tirely by  volcanic  extrusion  without  the  aid  of  erosion  or  secondary 
deformation.  The  larger  mountain  ranges  are  sculptured  in  rocks 
which  have  undergone  secondary  deformation  and  uplift.  They 
are  commonly  dated  from  the  time  of  deformation  and  uplift, 
rather  than  from  the  period  of  erosion.  Depending  on  the  nature 
of  the  deformation,  they  are  called  block  fault  mountains,  mono- 
clinal  fold  mountains,  fan  fold  mountains,  etc.,  though  it  has  been 
recognized  that  erosion  has  been  an  important  factor  in  causing 
the  present  topography.  Uplift  relative  to  sea  level  must  precede 
erosion  and  in  that  sense  is  primary  and  essential  to  mountain 
building.  The  uplift,  however,  may  produce  a  plateau  or  other 
forms  quite  different  from  mountains.  Differential  erosion  there- 
fore is  necessary  to  produce  the  forms  of  mountains.  In  time 
erosion  completely  base-levels  mountains,  as  it  has  so  largely  in 
pre-Cambrian  areas.  In  the  highest  existing  mountains  the  up- 
lift and  deformation  have  been  of  recent  date  and  erosion  has  not 
had  time  to  reduce  them. 

The  structure  of  the  greater  number  of  mountains  is  clearly 
the  result  of  tangential  shortening  of  the  earth's  crust  expressed 
in  folding  and  overthrust  faulting.  They  exist  in  chains  of  elon- 
gated ridges,  lying  end  to  end,  or  overlapping.  They  afford 
marked  evidence  of  greater  shortening  normal  to  the  general  trend 
of  the  chain  than  parallel  to  it.  Certain  of  the  folds  show  an 
irregular  dome-shape  and  seem  to  have  been  shortened  more  or  less 
equally  from  all  sides. 

Attention  is  here  especially  directed  to  mountains  developed  by 
differential  erosion  of  rocks  which  have  undergone  secondary 
deformation.  They  are  conspicuous  surface  expressions  of  the 
structures  described  in  the  earlier  pages  of  this  book  and  this 
discussion  will  therefore  be  brief. 

136 


MOUNTAINS   AND   FAULTS  137 

MOUNTAINS  AND   NORMAL  FAULTS 

Several  mountain  ranges  are  the  result  dominantly  of  nearly 
vertical  movements  along  faults.  Examples  of  these  are  found 
among  the  Great  Basin  ranges  of  the  West  where  there  are  faults 
with  a  displacement  of  over  a  mile  and  in  the  Wasatch  and  Sierra 
Nevada  mountains.  The  present  topography  of  the  Great  Basin 
ranges  is  due  partly  to  fault  scarps,  more  or  less  modified  by  ero- 
sion. There  is  a  difference  of  opinion  among  geologists  as  to  the 
relative  importance  of  the  two  factors  of  faulting  and  erosion. 
The  published  discussion  of  the  subject  is  of  general  interest,  as 
illustrating  the  trend  from  an  earlier  emphasis  on  structural  fea- 
tures, such  as  faults,  toward  a  wider  recognition  of  the  importance 
of  erosion.  (See  pp.  57-59.) 

MOUNTAINS  AND   THRUST  FAULTS 

While  thrust  faulting  has  played  an  important  part  in  the 
deformation  of  many  mountain  ranges  and  the  faults  influence  the 
present  topography,  erosion  has  so  modified  the  fault  topography 
that  it  is  difficult  to  state  in  simple  terms  the  influence  of  faulting 
in  producing  this  topography.  In  general  fault  slices  piled  one  on 
top  of  another  tend  to  form  the  present  elevations.  This  is  con- 
spicuously illustrated  in  the  Highlands  of  Scotland,  in  the  Scan- 
dinavian Highlands,  in  parts  of  the  Alps,  and  in  the  southern 
Appalachians.  Erosion,  working  on  the  tilted  fault  slices,  leaves 
linear  ridges  generally,  but  not  closely,  parallel  to  the  fault  traces, 
but  the  varying  hardness  of  the  rocks  and  the  physiographic 
conditions  play  such  an  important  part  that  there  is  usually  no 
close  relation  between  the  mountain  range  and  the  fault  traces. 
Such  ridges  tend  to  be  steeper  on  the  side  toward  which  the  over- 
thrust  is  moving  and  gentler  on  the  other  side.  The  fault  traces 
naturally  are  exposed  on  the  steep  faces. 

MOUNTAINS  AND   FOLDS 

Where  the  folds  are  somewhat  simple  and  open  there  is  a  dis- 
tinct tendency  for  erosion  to  cut  down  the  anticlines,  leaving  the 
synclines  as  ridges  between.  Synclinal  mountains  thus  formed  are 
well  illustrated  in  the  Appalachian  region.  The  stumps  of  moun- 
tains throughout  the  pre-Cambrian  are  largely  of  this  type.  The 


138  STRUCTURAL   GEOLOGY 

iron  "  ranges  "  of  Lake  Superior,  which  are  really  stumps  of  moun- 
tains, are  prevailingly  synclinal.  Less  frequently  the  anticline 
stands  as  a  topographic  elevation.  Illustrations  of  this  may  be 
seen  in  the  Appalachian  mountains.  This  structure  is  more  com- 
mon where  the  anticline  has  a  core  of  igneous  rock,  as  in  the  Front 
Range  of  Colorado. 

As  the  folding  becomes  closer  and  more  complicated,  the  rela- 
tions to  topography  likewise  are  more  complicated.  The  great 
overthrust  folds  or  "decken"  structure  of  the  Alps  and  to  a  less 
extent  some  of  those  of  the  southeastern  Appalachians  bordering 
the  Piedmont,  illustrate  this  complexity  of  relations.  The  general 
effect  is  to  pile  up  strata  in  the  same  manner  as  in  overthrust 
faults,  forming  ridges,  which  in  general  mark  the  present  elevations, 
but  the  varying  resistance  of  the  rocks  to  erosion  from  various 
causes  results  in  wide  variations  in  topography.  As  in  the  case 
of  thrust  faults,  the  steep  slopes  tend  to  be  on  the  side  away  from 
the  thrust;  in  gentle  slopes,  toward  the  thrust. 

In  an  area  of  monoclinal  folding  the  softer  beds  are  eroded  and 
the  more  resistant  beds  stand  out  as  linear  ridges  with  steep  sides 
generally  in  the  direction  opposite  to  the  dip  and  with  gentler 
slopes  in  the  direction  of  the  dip.  Somewhat  regular  step  moun- 
tains or  step  topography  may  be  produced  in  this  fashion. 

MORE    COMPLEX     RELATIONS    OF    MOUNTAINS    TO 

STRUCTURE 

The  above  statements  express  but  crudely  some  of  the  simpler 
relations  between  structure  and  mountain  ranges.  In  most  moun- 
tain ranges  there  have  been  repeated  deformations  and  uplifts  and 
repeated  cycles  of  erosion  which  leave  the  present  topography  in 
relations  to  structure  which  cannot  be  as  simply  stated  as  above. 
It  has  often  been  possible  to  work  out  the  complex  history  of  the 
relations  between  structure  and  erosion  in  the  development  of  the 
present  topography,  but  this  has  been  primarily  the  field  of  the 
physiographer  and  will  not  be  entered  into  here. 

LOCALIZATION   OF  MOUNTAINS 

Mountains  due  to  deformation  are  located  where  folding  and 
faulting  accompanied  by  uplift  result  from  failure  of  the  earth's 


LOCALIZATION   OF  MOUNTAINS  139 

shell.  The  part  of  differential  erosion  is  in  one  sense  secondary 
and  modifying.  The  highest  mountain  chains  are  those  of  recent 
age,  which  erosion  has  not  yet  had  time  to  cut  down.  The  older 
rocks,  principally  the  pre-Cambrian,  show  deeply  eroded,  folded, 
and  faulted  stumps  of  mountains.  Suess  has  emphasized  the 
extreme  deformation  of  the  pre-Cambrian  rocks  and  implies  that 
the  Archean  was  a  greater  mountain-building  era  than  any  era 
since.  It  is  apparent,  however,  that  the  Archean  has  suffered 
deformation  not  only  during  Archean  time  but  during  all  succes- 
sive periods.  Consequently  it  shows  in  general  more  folding  than 
the  rocks  of  later  periods,  but  it  does  not  follow  that  this  excessive 
amount  of  deformation  was  accomplished  during  the  pre-Cambrian, 
rather  than  during  later  periods.  While  it  is  entirely  conceivable 
that  the  Archean  may  have  been  a  time  of  mountain  building  on  a 
far  greater  scale  than  any  succeeding  period,  the  writer  doubts 
whether  this  has  been  established  on  an  inductive  basis. 

Mountain  areas  of  earlier  periods  have  commonly  been  the  locus 
of  mountain  building  in  later  periods.  Some  zones  of  weakness 
seem  to  have  been  permanent  through  much  of  geologic  history. 
Many  of  the  principal  mountain  chains  are  the  result  of  repeated 
foldings  and  uplifts  along  the  same  general  zone.  There  has  been  a 
tendency  also  for  successive  deformations  to  widen  the  moun- 
tainous zone. 

Many,  in  fact  most,  of  the  great  mountain  chains  are  near  the 
margin  of  continents.  Some  mountain  chains  which  do  not  now 
border  continents  did  so  at  the  time  of  their  deformations.  It  has 
long  been  recognized  that  mountains  have  developed  at  various 
periods  in  geologic  history  along  geosynclinal  shores  of  heavy  dep- 
osition. Thus  the  Appalachian  mountains  developed  along  the 
shore  area  of  heaviest  deposition  of  the  Paleozoic  sediments  against 
the  old  pre-Cambrian  Appalachia,  now  represented  in  part  by  the 
Piedmont  plateau. 

The  distribution  of  mountain  chains  along  continental  margins 
suggests  crowding  between  oceanic  and  continental  segments 
of  the  globe.  Chamberlin1  considers  such  crowding  to  be  due  to 
the  settling  of  the  larger  and  more  dense  oceanic  segments  as  a 
whole,  crowding  smaller  and  less  dense  continental  segments 
laterally  and  possibly  upward,  and  localizing  deformation  near 

1  Chamberlin,  T.  C.,  and  Salisbury,  R.  D.,  Geology,  Vol.  1,  1904,  p.  521. 


140  STRUCTURAL   GEOLOGY 

continental  margins.  According  to  the  advocates  of  the  theory  of 
isostasy,  this  crowding  is  due  to  the  readjustments  necessary  to 
restore  equilibrium  between  regions  of  different  density  when  this 
equilibrium  has  been  disturbed  by  transfers  of  material  by  erosion, 
or  by  any  other  agency.  By  others  the  localization  of  mountains 
in  these  zones  has  been  referred  more  or  less  vaguely  to  a  rise  of 
the  isogeotherms  into  the  base  of  the  thick  mass  of  sediments 
deposited  in  a  geosyncline,  softening  and  weakening  them,  and 
thereby  localizing  deformation  by  general  earth  stresses,  whatever 
their  origin.  The  causes  of  earth  movements  are  discussed  in  a 
subsequent  section.  The  foregoing  is  merely  to  notice  the  localiza- 
tion of  mountains  by  crowding  near  continental  margins. 

SUGGESTIONS    FOR   LABORATORY    STUDY   OF 
MOUNTAINS 

To  what  extent  may  the  topography  be  said  to  be  dominantly  influenced 
by  folding  or  faulting  or  other  secondary  rock  structures  in  the  following 
areas : 

In  the  southern  Appalachians:  See  U.  S.  Geological  Survey  folios,  es- 
pecially Monterey,  Va.  (folio  No.  61),  Cranberry,  N.  C.  (folio  No.  90), 
and  Rome,  Ga.  (folio  No.  78). 

In  the  Alps:  See  Mechanismus  der  Gebirgsbildung,  by  Albert  Heim, 
1878,  and  Geologische  Probleme^des  Alpengebirges,  by  G.  Steinmann: 
Zeitschrift  des  Deutschen  und  Osterreichischen  Alpenvereins,  Vol.  37, 
1906. 

In  the  Highlands  of  Scotland:  See  The  geological  structure  of  the  north- 
west Highlands  of  Scotland,  Mem.  Geol.  Survey,  Great  Britain,  1907. 

In  the  Great  Basin  region:  See  origin  and  structure  of  the  Basin  Ranges, 
by  J.  E.  Spurr:  Bull.  Geol.  Soc.  Am.,  Vol.  12,  1901,  pp.  217-270;  also 
U.  S.  Geological  Survey  folios  on  this  region. 

In  the  Rocky  Mountains:  See  stratigraphy  and  structure,  Lewis  and 
Livingston  Ranges,  Montana,  by  Bailey  Willis:  Bull.  Geol.  Soc.  Am., 
Vol.  13,  1902,  pp.  305-352,  and  the  following  U.  S.  Geological  Survey 
folios:  Spanish  Peaks,  Colo,  (folio  No.  71),  Sundance,  Wyo.  (folio  No.  127), 
Three  Forks,  Montana  (folio  No.  24),  Livingston,  Montana  (folio  No.  1), 
Little  Belt  Mountains,  Montana  (folio  No.  56). 

In  the  Ozarks:  Tahlequah,  Ind.  Terr.,  geologic  folio  (No.  122). 


MAJOR  UNITS  OF  STRUCTURE 

Geanticlines,  Geosynclines,  Ocean  Basins,  Continents,  Plateaus, 
Positive  and  Negative  Elements 

In  addition  to  the  secondary  rock  structures  and  their  expres- 
sions in  mountains,  discussed  in  previous  pages,  we  have  to  con- 
sider certain  larger  secondary  earth  structures  not  ordinarily 
within  the  range  of  our  detailed  observation  or  mapping.  These 
are  continents,  plateaus,  ocean  basins,  geanticlines,  geosynclines, 
positive  and  negative  elements. 

The  major  units  of  structure  of  the  kind  indicated  in  the 
above  heading  require  no  definition,  with  the  possible  exception  of 
geanticlines  and  geosynclines,  and  positive  and  negative  elements. 
Geanticlines  are  merely  anticlines  affecting  a  large  area.  They 
differ  only  in  size  from  anticlines,  and  the  delimiting  size  is  indef- 
inite. Willis1  has  used  the  name  "  positive  element "  for  portions 
of  the  earth's  crust  which  have  tended  during  geological  time  to 
rise  and  thereby  remain  uncovered  by  marine  sediments,  as  con- 
trasted with  " negative  elements"  which  have  been  submerged 
again  and  again  during  geologic  history.  The  pre-Cambrian  shield 
of  North  America  is  a  positive  unit;  the  Paleozic  area  of  the  Missis- 
sippi Valley  is  a  negative  element.  These  divisions  are  necessarily 
vague  and  their  boundaries  have  shifted  widely  during  geologic 
time. 

SHAPES  OF  MAJOR  ELEMENTS  OF  STRUCTURE 

A  notable  expression  of  the  common  tendency  toward  generaliza- 
tions from  complex  facts  is  the  frequent  attempt  of  geologists  to 
read  into  the  lineaments  of  the  earth's  surface  patterns  correspond- 
ing to  hypotheses  of  the  origin  of  the  earth  or  earth  deformation. 
One  of  the  best  known  early  attempts  at  this  was  the  so-called 

1  Willis,  B.,  A  theory  of  continental  structure  applied  to  North  America:  Bull. 
Geol.  Soc.  Am.,  Vol.  18,  1907,  p.  390. 

141 


142  STRUCTURAL   GEOLOGY 

tetrahedral  theory  of  the  earth.  A  tetrahedron  is  a  solid  body 
which  possesses  the  greatest  possible  surface  for  a  given  volume. 
On  the  hypothesis  that  the  earth's  interior  is  molten  and  is  cooling 
more  rapidly  than  its  shell,  it  was  assumed  that  the  shell  would 
tend  to  maintain  the  largest  possible  area  of  surface  and  therefore 
might  take  on  tetrahedral  lineaments.  Continental  areas  and 
mountain  chains  would  then  correspond  roughly  to  the  angles  and 
corners  of  the  tetrahedron.  By  standing  a  tetrahedron  on  one  of 
its  corners  and  calling  this  point  the  south  pole,  the  three  upper 
corners  and  angles  are  supposed  to  correspond  to  the  land  areas 
surrounding  the  north  pole.  The  three  angles  extending  down 
toward  the  south  polar  point  would  correspond  to  the  continental 
ridges  of  South  America,  Africa,  and  Australasia.  The  dominance 
of  the  land  area  in  the  northern  half  of  the  continent  would  accord 
with  the  dominance  of  projections  in  the  upper  half  of  the  tetra- 
hedron. It  is  needless  to  say  that  this  comparison  requires  some 
imagination.  It  is  cited  merely  as  illustrative  of  the  several 
hypotheses  offered.  Equally  good  comparisons  have  been  made 
with  other  geometric  forms. 

A  more  recent  generalization  is  that  of  Chamberlin,1  who  sug- 
gests that  the  great  negative  elements  of  the  earth,  represented 
largely  by  sea  areas — the  master  segments — should  be  expected 
to  have  polygonal  outlines  corresponding  to  the  primary  place 
assigned  them;  that  the  smaller  positive  segments  or  continental 
areas  left  between  these  major  segments  might  be  expected  to  have 
triangular  outlines,  or  at  least,  fewer  angles  than  the  major  con- 
trolling segments.  This  hypothesis  allows  of  a  greater  variety  of 
shapes  and  it  is  easier  to  conceive  that  continents  and  sea  areas 
conform  roughly  to  these  outlines. 

When  smaller  features,  such  as  mountain  chains,  are  considered, 
the  linear  distribution,  more  or  less  near  and  parallel  to  contacts 
of  positive  and  negative  elements,  is  obvious.  As  one  notes  the 
great  extent  and  persistence  of  these  linear  elements  and  notes  the 
synchronism  of  like  deformation  over  large  areas,  he  cannot  but 
suspect  that  the  major  earth  deformation  as  a  whole  may  ulti- 
mately be  reduced  to  simpler  terms  than  a  casual  inspection  of  the 
irregularities  of  the  surface  might  suggest. 

1  Chamberlin,  T.  C.,  and  Salisbury,  R.  D.,  Geology,  Vol.  1,  pp.  521-522. 


MAJOR   STRUCTURES  143 

ACTUAL  AND  APPARENT  UPLIFTS 

Actual  uplift  of  the  earth's  crust  may  come  about  through  the 
rigidity  of  the  crust,  allowing  tangential  thrust  to  be  transformed 
into  uplift,  or  through  increase  in  volume..  It  seems  to  be  demon- 
strated that  the  crust  is  rigid  only  on  a  small  scale  (see  p.  145), 
and  that  actual  uplift,  due  to  rigidity,  can  affect  only  a  small  area. 
Uplift  due  to  increase  in  volume  may  be  shown  also  to  have  very 
narrow  limits.  The  larger  uplifts  are  probably  apparent,  not 
actual,  and  may  be  caused  by  the  lowering  of  sea  levels  brought 
by  sinking  of  earth  segments.  In  other  words,  the  earth  move- 
ments are  dominantly  centripetal,  and  of  varying  intensity,  with 
the  result  that  certain  areas  appear  to  rise. 


ULTIMATE  FORCES  OF  SECONDARY  DEFORMATION 

OUTLINE   OF   PRINCIPAL   THEORIES 

Secondary  structures,  both  on  a  large  and  small  scale,  are  essen- 
tially the  result  of  failure  of  the  earth's  crust,  and  are  indeed 
evidence  of  this  failure.  Depending  somewhat  on  one's  point  of 
view  with  reference  to  the  origin  of  the  earth,  the  stresses  causing 
this  failure  have  been  ascribed  to  the  cooling  of  a  thin  shell  around 
a  liquid  core;  to  the  redistribution  of  temperatures  in  a  solid 
earth,  heat  from  the  center  moving  to  the  outer  portion  faster 
than  radiated  from  the  outer  portion  into  space;  to  erosion  causing 
a  disturbance  of  equilibrium  between  different  segments,  thereby 
releasing  the  potential  energy  available  in  differences  of  density  in 
adjacent  masses;  and  to  other  causes.  A  consideration  of  these 
forces  involves  a  discussion  of  hypotheses  of  the  origin  of  the  earth, 
which  it  is  not  the  purpose  here  to  attempt.  We  are  concerned 
primarily  with  the  manner  in  which  these  forces  are  localized  and 
directed,  not  with  the  ultimate  sources  of  the  stresses. 

Stripped  of  detail  and  modifying  considerations  there  appear  to 
be  two  main  hypotheses  to  account  for  deformation  of  the  earth's 
shell. 

First:  The  cooling  and  shrinking  of  the  nucleus  faster  than  the 
shell  causes  the  shell  to  collapse.  In  collapsing,  strong  tangential 
thrusts  are  set  up;  the  rocks  become  deformed  primarily  by  these 
thrusts,  and  subordinately  by  local  tensional  stresses  near  the 
surface.  Notwithstanding  this  failure  as  a  whole,  it  is  conceived 
that  the  rocks  are  sufficiently  rigid  to  transmit  thrusts  for  long 
distances,  developing  and  maintaining  by  their  rigidity  not  only 
mountain  ranges,  but  geanticlines  and  geosynclines,  plateaus,  con- 
tinents, oceanic  basins,  and  other  large  units  of  structure.  This  is 
the  old,  and  present  popular,  conception  of  earth  deformation. 

Second :  Deformation  based  on  quite  a  different  principle  is  that 
which  results  from  the  disturbance  of  isostatic  equilibrium  between 
the  segments  of  the  earth  which  are  of  different  density.  So  far 

144 


ULTIMATE   FORCES  145 

as  the  different  parts  of  the  earth  are  in  isostatic  equilibrium,  the 
transfer  of  loads  by  erosion  from  light  to  heavy  segments  may  so 
disturb  this  balance  between  heavy  and  light  segments  as  to  cause 
a  compensating  flow  of  rock  material  beneath  the  surface,  resulting 
in  rock  deformation.  This  principle  of  deformation  is  independent 
of  that  postulated  in  the  preceding  paragraph,  but  the  two  may  be 
combined  in  any  ratio;  one  does  not  necessarily  exclude  the  other. 
The  first  hypothesis  emphasizes  the  strength  of  rocks,  the 
second,  weakness  of  rocks.  The  first  hypothesis  in  its  simpler 
features  is  sufficiently  well  known  not  to  require  further  elucida- 
tion here.  The  second  is  discussed  below. 


ISOSTASY 

SUPPORT  OF  HYPOTHESIS  BY  RECOGNITION  OF  WEAKNESS  OF 

ROCKS 

In  the  past  the  strength  and  rigidity  of  rock  masses  was  supposed 
to  be  sufficient  to  develop  and  maintain  major  elevations  and  de- 
pressions of  the  earth's  surface.  It  was  supposed  that  the  shorten- 
ing of  the  earth's  crust  necessarily  accounted  for  the  lifting  of  great 
areas,  possibly  even  of  continental  areas,  on  the  arch  principle. 
Gradually  it  came  to  be  realized  that  this  was  demanding  too  much 
of  the  strength  of  rocks — that  rocks  in  large  masses  on  the  scale  of 
the  earth  are  weak.  Chamberlin  l  cites  calculations  to  show  that 
a  dome,  with  the  curvature  of  the  earth,  would  support  only  ^  of 
its  own  weight. 

The  recognition  of  the  weakness  of  rocks  favored  the  wider 
acceptance  of  the  hypothesis  of  isostasy  to  explain  the  major  in- 
equalities in  the  earth's  surface,  namely  that  the  inequalities  are 
due  to  differences  in  density  of  the  rock  masses — low  density  of 
certain  rocks,  and  hence  greater  specific  volume,  making  them 
stand  higher  above  the  surface  than  rocks  in  adjacent  areas  with 
greater  density  and  hence  smaller  specific  volume.  Continental 
areas  as  a  whole,  then,  would  be  areas  with  rocks  of  low  density 
compensated  by  higher  elevations.  The  sea  bottoms  would 'be 
areas  of  high  densities  of  rocks  compensated  by  the  depressions. 

The  causes  of  the  differences  in  density  required  by  the  isostatic 

1  Chamberlin,  T.  C.,  and  Salisbury,  R.  D.,  Geology,  Vol.  1,  1904,  pp.  555-556. 


146  STRUCTURAL   GEOLOGY 

theory  are  not  material  to  the  discussion.  We  are  concerned  with 
proof  of  the  existence  of  the  differences  in  density.  Parenthetically 
it  may  be  remarked  that  Chamberlin  believes  that  according  to  the 
planetesimal  theory  of  the  formation  of  the  earth  the  effect  of 
differential  weathering  and  of  vulcanism  would  tend  continuously 
to  arrange  the  densities  in  the  growing  earth  in  their  present  dis- 
tribution.1 

BUTTON'S  AND   GILBERT'S  OBSERVATIONS  ON  ISOSTASY 

Button  2  proposed  this  theory  in  connection  with  his  study  of 
western  mountains.  Gilbert,3  analyzing  and  discussing  the  gravity 
determinations  of  Putnam  of  the  Coast  and  Geodetic  Survey,  con- 
cluded "the  measurements  of  gravity  appear  far  more  harmonious 
when  the  method  of  reduction  postulates  isostasy  than  when  it 
postulates  high  rigidity.  Nearly  all  the  local  peculiarities  of 
gravity  admit  of  simple  and  rational  explanation  on  the  theory 
that  the  continent  as  a  whole  is  approximately  isostatic,  and  that 
the  interior  plain  is  almost  perfectly  isostatic." 

HAYFORD'S  OBSERVATIONS  ON  ISOSTASY 

Many  more  observations  of  the  Coast  and  Geodetic  Survey  4 
under  the  immediate  charge  of  Mr.  John  F.  Hayford,  have  made  it 
possible  to  state  more  definitely  to  what  extent  any  large  portion 
of  the  United  States  meets  the  requirements  of  isostasy.  At  some 
hundreds  of  stations  in  the  United  States  the  deflection  of  the 
plumb  bob  from  the  astronomic  vertical  was  determined.  With  the 
aid  of  topographic  maps,  the  lateral  pull  upon  the  plumb  bob  by 
topographic  elevations  was  calculated,  without,  of  course,  assign- 
ing any  deficiency  of  density  to  the  elevated  areas.  The  calculated 
deflection  from  the  vertical,  under  the  influence  of  the  topography, 
was  in  each  case  found  to  be  much  larger  than  the  actually  observed 
deflection,  though  usually  in  the  same  direction.  The  obvious 

1  Chamberlin,  T.  C.,  and  Salisbury,  R.  D.,  Geology,  Vol.  2,  1906,  pp.  106-110. 

2  Button,  C.  E.,  On  some  of  the  greater  problems  of  physical  geology:  Bull.  Phil. 
Soc.  of  Wash.,  Vol.  11,  1889,  pp.  51-64. 

3  Gilbert,  G.  K.,  Notes  on  the  gravity  determinations  reported  by  Mr.  G.  R. 
Putnam:  Bull.  Phil.  Soc.  of  Wash.,  Vol.  13,  1895,  p.  73. 

4  Hayford,  John  F.,  The  figure  of  the  earth  and  isostasy  from  measurements  in 
the  United  States.    Washington,  1909.    Also  Supplementary  investigation  in  1909 
of  the  figure  of  the  earth  and  isostasy.    Washington,  1910,  and  The  effect  of  topog- 
raphy and  isostatic  compensation  upon  the  intensity  of  gravity.    Washington,  1912. 


ISOSTASY  147 

inference  was  that  there  is  a  counteracting  pull  downward  due  to 
excess  of  density  at  that  point;  in  other  words,  that  there  is  excess 
of  density  in  the  topographic  depressions  corresponding  to  de- 
ficiencies in  the  elevations.  The  following  quotation  is  from  Hay- 
ford: 

"The  logical  conclusion  from  the  study  of  the  geoid  contours  for 
the  United  States,  taken  in  connection  with  the  fact  already  noted 
that  the  computed  topographic  deflections  are  much  larger  than 
the  observed  deflections  of  the  vertical,  is  that  some  influence 
must  be  in  operation  which  produces  an  incomplete  counter- 
balancing of  the  deflections  produced  by  the  topography,  leaving 
much  smaller  deflections  in  the  same  direction.  ..." 

"Both  the  general  approximate  studies  for  the  whole  world  of 
the  necessary  effects  of  the  known  topography  in  producing  de- 
flections of  the  vertical,  and  the  detailed  exact  study  made  for  the 
United  States  alone,  by  means  of  computed  topographic  deflec- 
tions and  geoid  contours,  indicate  that  one  must  look  to  the  dis- 
tribution of  the  subsurface  densities  for  an  explanation  of  the  dis- 
crepancies between  observed  deflections  of  the  vertical  and  the 
deflections  which  must  inevitably  be  produced  by  the  topography. 
Moreover,  from  the  general  considerations  set  forth  in  the  preced- 
ing paragraphs,  it  seems  that  there  must  be  some  general  law  of 
distribution  of  subsurface  densities  which  fixes  a  relation  between 
subsurface  densities  and  the  surface  elevations  such  as  to  bring 
about  an  incomplete  balancing  of  deflections  produced  by  topog- 
raphy on  the  one  hand  against  deflections  produced  by  variation 
in  subsurface  densities  on  the  other  hand. 

The  theory  of  isostasy  postulates  precisely  such  a  relation  be- 
tween subsurface  densities  and  surface  elevations.  .  .  ." 

"Keeping  this  contrast  in  mind,  the  writer  believes  that  the 
stress-differences  in  and  about  the  United  States  have  been  so 
reduced  by  the  isostatic  compensation  that  they  are  less  than  one- 
twentieth  as  great  as  they  would  be  if  the  continent  were  main- 
tained in  its  elevated  position  and  the  ocean  floor  maintained  in 
its  depressed  position  by  the  rigidity  of  the  earth.  .  .  ." 

"It  is  certain,  from  the  results  of  this  investigation,  that  the 
continent  as  a  whole  is  closely  compensated,  and  that  areas  as 
large  as  States  are  also  closely  compensated.  It  is  the  writer's 
belief  that  each  area  as  large  as  one  degree  square  is  generally 


148  STRUCTURAL   GEOLOGY 

largely  compensated.  The  writer  predicts  that  future  investiga- 
tions will  show  that  the  maximum  horizontal  extent  which  a 
topographic  feature  may  have  and  still  escape  compensation  is 
between  1  square  mile  and  1  square  degree.  This  prediction  is 
based,  in  part,  upon  a  consideration  of  the  mechanics  of  the  prob- 
lem." 1 

EARTH  MOVEMENTS  IN  RELATION  TO  ISOSTASY 

When  one  keeps  in  mind  the  fact  that  erosion  is  continuously 
shifting  the  load,  and  that  there  is  local  evidence  of  the  erosion  of 
thousands  of  feet  of  sediments,  it  must  be  inferred,  if  there  is  the 
present  high  degree  of  isostatic  adjustment  postulated  above,  that 
the  process  of  isostatic  adjustment  is  a  continuous  one,  accom- 
plished by  deep-seated  rock  flow  keeping  pace  with  the  transporta- 
tion of  surface  material.  Movements  thus  initiated  should  cause 
other  movements,  principally  near  the  contacts  of  the  positive 
elements  of  low  density  with  the  negative  elements  of  high  density 
(see  p.  141).  Initial  dip  of  sediments  in  these  areas  would  still 
further  localize  deformation. 

ISOSTASY   IN   RELATION   TO   RIGIDITY  OF  ROCKS 

If  the  United  States,  as  well  as  certain  other  parts  of  the  world, 
is  in  a  state  of  isostatic  equilibrium  to  such  a  remarkable  degree, 
it  would  follow  that  the  major  irregularities  of  the  surface  are  not 
due  to  the  rigidity  of  the  rocks,  but  rather  to  their  weakness.  If 
their  rigidity  were  sufficient  to  account  for  the  irregularities,  there 
would  be  no  need  of  the  theory  of  isostatic  adjustment  to  explain 
them,  and  there  would  be  great  variations  from  such  a  state  of 
equilibrium.  Isostasy  and  rigidity  are  mutually  exclusive  on  any 
large  scale.  If  the  rocks  were  adequately  rigid,  it  would  be  im- 
possible for  them  to  yield  sufficiently  to  accomplish  a  delicate 
isostatic  adjustment.  But  rigidity  is  effective  to  some  extent, 
notwithstanding  this  tendency  toward  adjustment,  for  small 
units — to  what  extent  is  still  an  open  question. 

Rigidity  is  sufficient  to  account  for  periodicity  in  major  earth 
movements.  During  long  periods  of  quiet  the  rocks  seem  to  have 

1  Hayford,  John  F.,  The  figure  of  the  earth  and  isostasy  from  measurements  in 
the  United  States:  Coast  &  Geodetic  Survey,  Washington,  1909,  pp.  65,  66,  166,  and 
169. 


ISOSTASY  149 

been  sufficiently  rigid  to  have  allowed  the  enormous  stresses  to 
accumulate  which  found  expression  in  mountain-making  periods. 

DEPTH  OF  ISOSTATIC  COMPENSATION 

The  differences  in  density  postulated  by  isostasy  cannot  be  ex- 
pected to  extend  downward  indefinitely;  in  fact,  the  theory  was 
developed  to  accord  with  the  then  prevailing  notion  that  beneath 
the  solid  shell  was  a  liquid  or  a  near-liquid  substratum  upon  which 
the  shell  rested  or  floated.  The  depth  through  which  the  differences 
of  density  were  supposed  to  extend  has  been  called  the  depth  of 
compensation.  A  plane  at  this  depth  would  support  equal  weights 
of  material  above,  regardless  of  their  density;  below  this,  the  den- 
sity is  supposed  to  be  uniform.  Postulating  the  existence  of  such 
a  plane  of  complete  compensation,  Hayford  assumed  various 
arbitrary  depths  in  order  to  find  out  which  one  corresponded  most 
closely  to  the  facts  of  the  gravity  observations.  For  each  of  the 
arbitrary  depths  calculated,  three  alternative  distributions  of 
density  were  assumed — 1st,  uniform  distribution  of  density  to 
the  depth  of  complete  compensation;  2d,  a  gradually  diminishing 
difference  in  density  to  this  depth;  3d,  a  maximum  difference  in 
density  at  some  intermediate  point.  Depending  on  distribution 
of  density  chosen,  the  depth  of  complete  compensation  was  cal- 
culated to  be  between  60  and  150  miles.  With  a  uniform  distribu- 
tion of  density  a  depth  of  compensation  of  76  miles  was  found  best 
to  correspond  with  the  plumb  bob  observations.  The  discrep- 
ancies are  so  slight  that  Hayford  concludes  that  the  area  of  the 
United  States  falls  one-tenth  short  of  complete  isostatic  adjust- 
ment. 

CRITICISM   OF  THEORY  OF  ISOSTASY 

Unquestionably  the  plumb  bob  deflections  show  the  existence 
of  some  sort  of  isostatic  compensation,  the  higher  areas  having 
lower  density  and  the  lower  areas  having  higher  density;  but  it  is 
questionable  whether  the  compensation  is  as  complete  as  indicated 
by  Hayford.  Lewis  1  has  called  attention  to  the  fact  that  the  con- 
ception of  the  existence  of  a  plane  of  complete  compensation  by 
Hayford  is  an  assumption,  as  in  fact  is  so  stated  by  Hayford;  that 
having  found  the  depth  of  such  an  hypothetical  plane  which  would 

1  Lewis,  Harmon,  The  theory  of  isostasy:  Jour.  Geol.,  Vol.  19,  1911,  pp.  603-626. 


150  STRUCTURAL   GEOLOGY 

most  nearly  satisfy  the  requirements  of  the  inferences  from  ob- 
servation, it  is  not  permissible  in  turn  to  use  this  hypothetical 
depth  as  a  standard  against  which  to  measure  variations  from  the 
requirements  of  the  assumption  of  isostatic  compensation  at  this 
depth.  It  is,  in  effect,  reasoning  in  a  circle.  Lewis  showed  that 
similar  close  accord  with  the  observations  could  be  secured  by  as- 
suming partial  compensation  at  less  depths,  or  over-compensation 
at  greater  depths.  In  other  words,  while  the  facts  clearly  in- 
dicate some  sort  of  compensation,  they  do  not  so  clearly  dis- 
criminate between  complete  compensation,  under-compensation, 
and  over-compensation.  Rigidity  of  the  rocks  is  known  at  the 
surface  to  play  some  part — it  may  be  a  considerable  part — in 
preventing  complete  isostatic  adjustment.  In  so  far  as  it  is  im- 
portant, it  favors  the  assumption  of  under-compensation  or  over- 
compensation,  rather  than  complete  compensation. 

Hayford's  reply  to  this  argument  is  that  the  actual  detailed 
observations  and  computations  yield  results  more  nearly  accordant 
with  his  assumption  of  complete  compensation  than  with  assump- 
tions of  over-compensation  or  under-compensation.1 

From  the  geological  standpoint  there  are  difficulties  in  the  way 
of  the  complete  acceptance  of  the  theory  of  isostasy  because  of  the 
fact  that  areas  of  uplift  and  depression  or  areas  of  erosion  and 
deposition  have  not  been  continuously  such  during  geological 
history;  a  given  area  is  likely  to  be  one  of  alternate  uplift  and  de- 
pression and  of  alternate  erosion  and  deposition.  If  uplift  and  de- 
pression are  related  to  density,  as  assumed  by  the  isostatic  theory, 
these  alternations  of  uplift  and  depression  require  alternations  of 
states  of  density,  which  is  not  satisfactorily  explained  under  the 
isostatic  theory. 

There  seems  also  to  be  objection  on  the  ground  that  differences 
in  density  could  not  be  maintained,  especially  in  the  zone  of  rock 
flowage,  and  therefore  would  not  for  long  be  a  source  of  deforma- 
tion. If,  on  the  other  hand,  the  rocks  are  rigid  enough  to  maintain 
these  differences  in  density,  and  the  loading  of  the  denser  segments 
by  sedimentation  is  sufficient  to  start  movement  toward  the  lighter 
segments,  the  question  naturally  arises  as  to  the  reason  for  the 
absence  of  movement  in  the  opposite  direction  before  the  erosion 

1  Hayford,  John  F.,  Isostasy,  a  rejoinder  to  the  article  by  Harmon  Lewis:  Jour. 
Geol.,  Vol.  20,  1912,  pp.  562-578. 


ISOSTASY  151 

and  deposition  took  place.  If  there  was  isostatic  equilibrium  in 
the  first  instance,  then  at  some  point  above  the  plane  of  compensa- 
tion, whether  it  was  complete  or  partial,  stresses  must  have  been 
acting  from  the  lighter  and  higher  segments  toward  the  heavier 
and  lower.  According  to  the  theory  of  isostasy  rocks  are  rigid 
enough  to  prevent  this  actual  movement;  and  yet  it  is  argued  that 
movement  should  occur  when  the  situation  is  reversed  and  stresses 
of  equal  (or  less?)  magnitude  are  set  up  in  an  opposite  direction  by 
erosion  of  the  lighter  segments  and  deposition  on  the  heavier  ones. 

The  fact  of  high  areas  being  light  and  low  areas  being  dense  does 
not  necessarily  imply  that  the  difference  in  density  is  the  cause  of 
the  differences  in  elevation  or  deformation.  This  latter  may  be  an 
incidental  accompaniment  or  may  be  the  result  of  deformation  by 
thrust  or  gravity.  Deformation  of  rocks  under  thrust  or  gravity 
stresses  is  localized  in  the  weakest  places.  It  may  be,  then,  that 
the  light  areas  are  weaker  than  the  heavy  ones.  They  would  tend, 
therefore,  to  be  folded  and  crowded  up.  In  one  sense,  then,  the 
high  areas  may  be  high  because  they  are  light  and  weak;  but  this 
is  quite  a  different  conception  of  the  nature  and  causes  of  deforma- 
tion from  that  postulated  by  isostasy.  The  facts  cited  to  support 
isostasy  are  fully  as  well  in  accord  with  such  an  alternative  hypoth- 
esis of  deformation. 

Again,  it  is  possible  that  light  areas  are  light  because  they 
are  high,  and  not  high  because  they  are  light.  The  processes 
of  katamorphism,  which  increase  the  volume  and  decrease  the 
density  of  rocks,  affect  higher  areas  to  a  greater  extent  than  lower 
water-covered  areas.  This  is  undoubtedly  a  real  factor,  but 
whether  sufficiently  important  to  explain  any  considerable  part 
of  the  observed  differences  in  density  is  not  yet  known. 

The  inference  from  gravity  observations  that  high  areas  are 
generally  light,  applies  principally  to  broad  areas  of  uplift  and  not 
to  the  minor  units  of  structure.  The  highest  peaks  are  determined 
essentially  by  their  resistance  to  erosion  and  not  alone  by  their 
density.  In  certain  parts  of  central  Brazil  the  highest  peaks  are 
hard  hematite,  with  a  specific  gravity  of  5,  which  happens  to  be 
the  most  resistant  material  in  this  region.  These  particular  peaks 
would  not  be  explained  on  the  isostatic  principle,  but  when  taken 
in  connection  with  the  broad  area  of  uplift  of  which  they  are  a  part, 
the  principle  might  still  hold. 


152  STRUCTURAL   GEOLOGY 

It  may  be  concluded  that  a  condition  of  isostasy  exists,  but  to 
what  degree  is  still  a  matter  of  doubt.  The  disturbance  of  this 
condition  is  a  probable  factor  in  the  deformation  of  rocks,  but 
there  are  other  important  and  perhaps  more  important  factors. 

CAUSES  OF  TENSION 

In  the  above  discussion  of  major  causes  of  the  earth's  deforma- 
tion nothing  has  been  said  about  tension,  for  in  fact  the  major  de- 
formation of  the  earth  has  been  by  tangential  compression,  result- 
ing in  mountain  chains  and  overthrust  faults,  whereas  tension 
structures  have  been  usually  regarded  as  local  and  subsidiary. 
In  connection  with  the  discussion  of  tension  joints  and  tension 
faults  on  previous  pages  (see  pp.  22,  39)  local  conditions  causing 
tension  have  been  cited.  That  tension  is  present  on  any  large 
scale  is  not  certain. 

Neither  the  so-called  contractional  theory  of  earth  deformation 
or  the  theory  of  isostasy  discussed  above  imply  the  existence  of 
tension  in  our  zone  of  observation  as  anything  but  subsidiary  and 
consequent  upon  thrusts.  Under  the  contractional  theory  tension 
is  produced  in  the  earth's  shell  when  the  circumferential  shortening 
by  cooling  predominates  over  compression  and  thrust  in  the  shell 
due  to  radial  shortening.  At  the  surface  and  to  a  depth  of  a  few 
miles,  the  circumferential  contraction  by  cooling  is  at  a  minimum, 
whereas  thrust  due  to  collapse  of  the  shell  is  at  a  maximum. 
Deeper  below  the  surface  cooling  is  going  on  more  rapidly  and  it  is 
supposed  that  the  circumferential  shortening,  involving  tension, 
may  predominate  over  a  thrust,  though  at  this  depth  rock  flowage 
might  prevent  actual  tensional  openings.  At  some  intermediate 
depth,  called  the  level  of  "no  strain,"  it  has  been  presumed  that 
the  circumferential  shortening  just  equalized  the  thrust  due  to 
collapse  and  there  would  be  no  lateral  tension  or  compression. 
This  theory  therefore  implies  no  general  state  of  tension  within  our 
zone  of  observation. 

The  deformation  involved  in  the  disturbance  of  isostasy  like- 
wise does  not  imply  tension  except  locally. 

CONCLUSION  AS  TO  MAJOR  CAUSES  OF  DEFORMATION 

We  conclude  that  earth  deformation  is  principally  due  to 
gravity,  locally  transformed  into  thrust,  and  causing  a  collapse 


MAJOR   CAUSES   OF  DEFORMATION  153 

and  buckling  of  the  earth's  shell;  that  the  known  differences  in 
density  between  higher  and  lower  areas  indicate  some  sort  of  an 
isostatic  adjustment;  that  this  isostatic  adjustment  may  be  an 
accompaniment  or  result  of  mechanical  thrust,  or  that  it  may  be 
an  initial  condition,  the  disturbance  of  which  by  erosion  would 
cause  deformation,  independent  of  any  majcr  thrust  due  to  the 
collapse  of  the  earth's  shell;  that  tension  is  local  and  subsidiary  to 
thrust. 

This  conclusion  throws  some  emphasis  on  the  competence  of  the 
earth  to  transmit  thrusts  and  to  cause  and  sustain  large  uplifts. 
It  is  believed  that  this  is  possible:  (1st)  because  of  the  competence 
of  the  beds  of  the  zone  of  rock  fracture,  and  (2d)  because  of  the 
actual  squeezing  up  of  the  rock  material  from  below  in  the  zone 
of  flow,  this  squeezing  possibly  affecting  the  lighter  rather  than 
the  heavier  material.  There  seems  to  be  no  reason  why  the  crowd- 
ing together  of  material  by  rock  flowage  in  a  deep-seated  zone 
should  not  account  for  major  uplifts  in  which  the  surface  buckling 
seems  small,  as  in  the  Cascade  Range.  The  great  pressures  in  the 
zone  of  rock  flowage  may  impart  a  high  degree  of  rigidity  to  the 
mass  capable  of  transmitting  thrusts — in  spite  of  the  fact  that  the 
rock  flows. 

Whatever  the  cause  of  deformation,  it  is  apparent  that  the 
earth's  shell  is,  as  a  whole,  a  weak  or  failing  structure.  The 
secondary  structures  which  have  been  described  are  evidences  of 
failure.  Rigidity  has  not  prevented  failure  except  for  the  smallest 
units — it  has  postponed  failure,  and  favored  a  certain  periodicity 
to  earth  movements. 

LOCAL  AND  MINOR  CAUSES  OF  DEFORMATION 

Weathering  involves  increase  in  volume  of  some  rocks.  This 
increase  in  volume  sets  up  compressive  strains  sufficient  for  minor 
local  deformation.  Some  minor  folding  has  been  attributed  to 
this  cause.1 

The  purely  mechanical  effects  of  heating  and  cooling  at  the  sur- 
face are  known  to  produce  local  deformation.  (See  pp.  22,  25.) 

Removal  of  a  load  by  erosion  from  a  rock  under  compreesive 
strain  (for  whatever  cause)  may  give  sufficient  relief  to  allow  def- 

1  Campbell,  D.  F.,  Rock  folds  due  to  weathering:  Jour.  Geol.,  Vol.  14,  1906, 
pp.  718-721. 


154  STRUCTURAL   GEOLOGY 

ormation  of  the  rock.  It  is  not  uncommon  in  quarry  and  other 
underground  excavations  for  rocks  to  swell  and  buckle  when  the 
superimposed  pressure  is  removed. 

Unconsolidated  sediments  in  a  drift  may  be  deformed  when 
crowded  or  overridden  by  a  glacier.  Overthrust  folds  may  be 
thus  developed. 

RELATION  BETWEEN  DEFORMATION  AND  VULCANISM 

In  regions  of  igneous  rocks  evidences  of  rock  deformation  are 
likely  to  be  unusually  numerous  and  conspicuous.  For  instance, 
cleavage  is  sometimes  well  developed  in  rocks  which  have  been 
intruded  by  a  great  batholith,  as  in  the  Black  Hills  area  of  South 
Dakota.  Joints  and  faults  are  abundant  in  areas  of  volcanic 
activity  as  is  shown  in  the  maps  of  some  of  the  western  mining 
districts  (see  p.  43).  It  is  frequently  possible  to  infer  that  the 
faulting  closely  followed  and  perhaps  accompanied  the  intrusion 
of  the  igneous  rocks.  Shattering  of  wall  rocks  near  contacts  with 
intrusives  is  a  commonly  observed  phenomenon.  Presumably 
mechanical  pressures  and  temperature  changes  combine  to  produce 
this  result. 

Not  less  obvious  is  the  tendency  for  igneous  rocks  when  in- 
truded to  follow  pre-existing  joint  and  fault  planes  or  to  be  de- 
flected in  their  course  by  folds.  The  association  of  vulcanism  with 
mountains  is  well  known. 

Earthquakes  are  both  the  cause  and  result  of  rock  deformation. 
Some  earthquakes  are  related  to  vulcanism  both  in  time  and  place 
(see  page  70). 

These  various  relations  indicate  a  genetic  relationship  between 
secondary  structures  and  igneous  activity.  A  broader  view  of  the 
situation  is  that  both  vulcanism  and  the  development  of  secondary 
structures  are  closely  related  effects  of  great  earth  movements. 
It  has  been  shown  to  be  probable  that  deep  in  the  zone  of  flowage 
rocks  are  at  such  temperatures  that  they  would  liquefy  if  the  pres- 
sure upon  them  were  not  so  great.  A  change  of  conditions  result- 
ing from  any  great  earth  movement,  whatever  its  cause,  may 
tend  to  disturb  the  equilibrium  between  pressure  and  temperature 
and  allow  the  rock  to  liquefy.  Having  then  less  density  than  the 
unliquefied  rocks,  it  moves  upward. 


DEFORMATION   AND   VULCANISM  155 

From  this  point  of  view  both  vulcanism  and  secondary  deforma- 
tion are  the  results  of  great  readjustment  in  major  segments  of  the 
earth's  shell.  Looked  at  on  a  smaller  scale,  vulcanism  and  de- 
formation are  found  to  have  mutually  reacted,  with  the  result 
that  either  may  be  in  a  causal  relation  to  the  other,  as  in  the 
illustrations  given  above. 


UNCONFORMITY 

Contiguous  formations  are  said  to  be  unconformable  when  there 
is  evidence  of  an  erosion  interval  of  some  magnitude  between  their 
periods  of  formation  or  evidence  of  cessation  of  deposition  between 
them.  In  either  case  there  is  loss  of  part  of  the  geological  record. 
The  term  unconformity  is  sometimes  used  to  indicate  primarily 


FIG.  66.  Horizontally  bedded  limestone,  resting  unconformably  on  vertical  beds 
of  Proterozoic  quartzite.  Box  Canyon,  near  Ouray,  Colo.  After  R.  T.  Cham- 
berlin. 

the  physical  discordance;  sometimes  it  is  applied  principally  to  the 
time  interval  implied  by  the  discordance;  it  usually  implies  both. 
The  evidences  of  unconformity  cited  below  are  both  physical  and 
organic.  The  secondary  deformation  of  rocks  with  which  this 
book  is  mainly  concerned  is  only  one  of  the  factors  to  be  considered 
in  unconformity.  Stratigraphy,  physiography  and  paleontology 
are  others, — in  fact  adequate  understanding  of  the  significance  of 

156 


UNCONFORMITY 


157 


unconformity  involves  the  widest  range  of  geological  knowledge. 
The  subject  is  treated  here  principally  in  its  relation  to  structural 
geology,  and  not  in  the  broader  sense  that  it  is  required  for  a 
philosophical  understanding  of  its  significance.  Involving,  as  it 
does,  considerations  other  than  structural,  it  has  been  left  to  the 
last  chapter. 

IDENTIFICATION   OF   UNCONFORMITY 

Physical  evidences  of  unconformity  are: 

(1)  Evidence  of  erosion,  even  without  intervening  deformation 
between  formations. 

(2)  Difference  in  Metamorphism: — Stratigraphically  lower  rocks 
may  have  suffered  so  much  more  metamorphism  than  overlying 


FIG.  67.  Ideal  sketch  to  illustrate  unconformities.      After  Spurr. 
of  conformity;  B.  Later  line. 


A.  Earlier  line 


beds  of  similar  lithology  as  to  indicate  the  probability  of  a  time 
interval  between  them.  Original  differences  in  lithology  also 
influence  the  nature  and  extent  of  metamorphism.  This  fact 
should  not  be  overlooked. 

(3)  Difference    in    Deformation:  —  Stratigraphically    underlying 
rocks  may  be  folded  or  cracked  or  may  be  schistose  as  result  of 
flowage,  while  these  features  may  be  less  conspicuous  or  lacking  in 
upper  beds  of  similar  kinds,  indicating  a  time  interval  between 
their  periods  of  formation.    This  criterion  must  be  carefully  used, 
for  the  differences  in  deformation  may  be  due  simply  to  varying 
competence  of  the  different  beds. 

(4)  Difference  in  Number  of  Igneous  Intrusions:  —  Stratigraphi- 
cally underlying  beds  may  be  intruded  by  igneous  rocks,  which 
have  not  intruded  the  upper  beds.     This  may  not  in  itself  be 
evidence  of  unconformity,  but  may  confirm  other  evidences  of  the 


158  STRUCTURAL   GEOLOGY 

existence  of  an  erosion  interval  between  lower  and  upper  beds.  If 
the  igneous  rock  in  the  lower  bed  is  a  plutonic  rock  and  appears  on 
the  contact  erosion  plane,  it  is  evidence  that  the  erosion  interval 
has  been  of  sufficient  duration  to  allow  of  the  removal  of  a  great 
thickness  of  rock. 

(5)  Basal  conglomerate  in  the  upper  beds,  carrying  fragments 
from  the  rocks  beneath  the  contact  plane.    If  this  conglomerate 
contains  a  variety  of  fragments  derived  from  a  considerable  area, 
it  is  more  significant  of  a  time  interval  perhaps  than  a  conglomerate 
made  up  of  fragments  entirely  like  the  immediately  underlying 
rock.     However,  if  the  underlying  rocks  are  homogeneous  over 
great  areas,  the  overlying  basal  conglomerates  may  show  a  marked 
homogeneity  of  fragments.     Intraformational  conglomerates  are 
sometimes  formed  by  exceptional  storms  or  other  causes,  in  the 
course  of  a  continuous  deposition  of  sediments.    Such  conglomer- 
ates mark  no  erosion  interval  of  magnitude  and  have  little  signif- 
icance with  reference  to  unconformity. 

While  a  basal  conglomerate  indicates  unconformity,  the  absence 
of  such  a  conglomerate  does  not  disprove  unconformity,  for 
students  of  sedimentation  now  find  many  conditions  under  which 
sediments  may  be  deposited  unconformably  on  an  older  surface 
without  intervening  conglomerates.  The  base  of  the  Paleozoic 
in  the  Mississippi  Valley  as  a  whole  is  remarkably  free  from  basal 
conglomerates  except  near  monadnocks  on  the  old  pre-Cambrian 
peneplain.  The  Niagara  limestone  resting  on  the  pre-Cambrian 
rocks  of  the  Cobalt  district  of  Ontario  furnishes  a  fine  example  of 
unconformable  contact  without  basal  conglomerate. 

(6)  Field  relations  and  areal  distribution  of  rocks  may  indicate 
an  unconformity  even  where  actual  contacts  or  other  evidences  are 
lacking.    For  instance,  a  continuous  bed  of  quartzite  lying  along- 
side of  a  heterogeneous  group  of  rocks  with  irregular  distribution 
would  in  itself  suggest  unconformity  between  these  rocks  and  the 
quartzite.    This  criterion  of  field  relations  is  of  the  utmost  practical 
importance.    It  is  frequently  possible  from  a  preliminary  study  of 
maps  showing  areal  distribution  of  lithologic  types  to  infer  possible 
unconformities,  and  if  so,  to  direct  further  field  work  with  much 
greater  effectiveness  than  would  otherwise  be  possible. 

(7)  Difference  in  lithology;  as,  for  instance,  where  a  sedimentary 
rock  rests  upon  an  igneous  rock  without  intrusive  relations. 


UNCONFORMITY  159 

(8)  There  may  be  an  irregular  erosion  surface  separating  parallel 
strata.    Differences  in  lithology  on  the  two  sides  of  the  contact  or 
fossil  evidence  may  aid  in  determining  this  surface. 

(9)  Hiatus  in  the  fossil  record  between  successive  beds. 

(10)  Absence  of  rocks  between  successive  beds  known  elsewhere 
to  have  been  deposited  in  this  relation. 

Commonly  the  greater  number  of  these  criteria  can  be  used  in 
working  out  unconformity.  One  line  of  evidence  can  usually  be 
substantiated  by  others. 

INTERPRETATION  OF  UNCONFORMITY 

Unconformity  represents  a  lost  interval  not  otherwise  recorded 
at  that  place.  This  lost  interval  may  involve  (a)  a  cessation  of 
deposition,  usually  involving  emergence,  and  often  accompanied 
by  deformation  of  the  rocks;  (b)  denudation,  usually  by  subaerial 
processes;  (c)  resumption  of  deposition,  usually  following  sub- 
mergence, but  often  by  terrestrial  processes.1 

The  appraisement  of  the  value  of  an  unconformity  requires  much 
care.  The  terms  " great"  and  " slight"  frequently  applied  to 
unconformity,  express  the  value  very  crudely.  By  great  uncon- 
formity may  be  meant  one  in  which  there  is  a  prominent  discord- 
ance of  structure,  or  one  indicating  the  absence  of  great  thicknesses 
of  strata,  or  a  long  lapse  of  time,  or  any  combination  of  these 
features.  Usually  it  is  intended  to  imply  that  the  discordance  is 
pronounced  and  that  there  is  a  great  loss  of  record.  It  is  desirable, 
wherever  possible,  that  these  factors  be  discriminated,  even  though 
their  quantitative  value  cannot  be  determined. 

The  study  of  unconformities  broadly  as  continental  features  is  of 
significance  to  structural  geology  as  indicating  the  major  warpings 
and  oscillations  of  the  continent  with  reference  to  the  sea.  If 
oceanic  basins  have  been  permanent  during  geological  time,  it  may 
be  supposed  that  there  are  no  unconformities  indicated  by  strata 
there  deposited.  However  meager,  the  record  may  be  one  of  con- 
tinuous deposition.  The  continents,  however,  from  the  beginning 
of  the  geological  record  have  always  in  some  part  stood  above 
water,  have  in  some  part  been  undergoing  erosion,  and  therefore 

1  Blackwelder,  Eliot,  The  valuation  of  unconformities:  Jour.  Geol.,  Vol.  17, 
1909,  p.  290. 


160 


STRUCTURAL   GEOLOGY 


fall  short  of  a  complete  record  of  deposition.  By  migrating  from 
place  to  place  during  continental  movements,  animals  might 
conceivably  have  lived  continuously  on  the  erosion  surfaces  which 
marked  unconformities  in  the  geologic  record.  Thus  it  appears 


Quaternary 

Tertiary 

Cretaceous 


Silurian 

Ordovician 

Cambrian 


FIG.  68.  Diagram  of  an  unconformity  with  lateral  extensions  and  restrictions. 
After  Blackwelder.  The  extent  and  duration  of  the  principal  periods  and 
areas  of  sedimentation,  with  their  corresponding  rock  systems,  are  shown  in 
solid  black.  The  white,  on  the  other  hand,  denotes  the  time  and  extent  of 
erosional  conditions  and  corresponding  unconformities. 

that  in  one  sense  unconformities  are  continuous  physically  and 
chronologically;  but  they  shift  back  and  forth  across  the  continents 
with  successive  oscillations  and  inundations.  It  is  equally  true 
that  any  localized  unconformity  is  represented  somewhere  else  by 
a  continuous  record  of  deposition.  As  Blackwelder  states  it:  "The 
entire  geologic  record,  then,  is  not  to  be  conceived  of  as  a  pile  of 


UNCONFORMITY  161 

strata,  but  as  a  dovetailed  column  of  wedges,  the  unconformities 
and  rock  systems  being  combined  in  varying  proportions.  The 
former  predominate  in  some  places  and  periods,  while  the  latter 
prevail  in  others."  1 

SUGGESTIONS  FOR  LABORATORY  STUDY  OF  UNCON- 
FORMITY 

1.  What  different  kinds  of  contacts,  and,  therefore,  different  relations 
between  rock  masses,  can  be  found  on  the  Three  Forks  and  Livingston, 
Montana,  geologic  maps?  (geologic  folios  U.  S.  Geol.  Survey,  Nos.  24 
and  1). 

2.  What  different  kinds  of  field  evidence  for  unconformity  can  be 
found  on  the  following  geologic  maps:  Three  Forks,  Montana  (geologic 
folio  No.  24,  U.  S.  Geol.  Survey),  Holyoke,  Mass,  (geologic  folio  No.  50, 
U.  S.  Geol.  Survey),  Milwaukee,  Wis.  (geologic  folio  No.  140,  U.  S.  Geol. 
Survey),  Mount  Stuart,  Wash,  (geologic  folio  No.  106,  U.  S.  Geol.  Survey), 
Hartville,  Wyo.  (geologic  folio  No.  91,  U.  S.  Geol.  Survey),  maps  of  the 
Mesabi,  Gogebic  and  Marquette  districts  of  Lake  Superior  (Mon.  52,  U. 
S.  Geol.  Survey). 

3.  The  historical  significance  of  various  unconformities: 

a.  On  the  Hartville,  Wyo.,  geologic  map  (geologic  folio  No.  91,  U.  S. 
Geol.  Survey)  determine  the  stratigraphic  hiatus  in  terms  of  forma- 
tions, at  several  different  points  in  the  district. 

b.  The  same  for  relative  degree  of  discordance  between  the  beds. 

c.  By  studying  the  geologic  maps  in  the  following  U.  S.  Geological 
Survey  folios  determine  as  closely  as  possible  the  time  value  of  the 
unconformity  at  the  base  of  the  coastal  plain  in  eastern  United 
States:  Trenton,  N.  J.  (folio  No.  167),  Washington,  D.  C.  (folio 
No.  70),  Mercersburg-Chambersburg,  Pa.  (folio  No.  170),  Rome, 
Ga.  (folio  No.  78),  and  Knoxville,  Tenn.  (folio  No.  16). 

1  Op.  Cit.,  p.  299. 


INDEX 


Adams,  F.  D.,  4,  5,  6,  7,  9,  10,  11.  88, 
89,  90,  92,  100,  102 

Adams,  L.  H.,  76 

Adirondack  graphite  deposits,  sedi- 
mentary origin  of,  99 

Alabama.     See  Gadsden 

Alaska,  earthquakes  at  Yakutat  Bay, 
70;  fault  scarp,  58;  folded  schist, 
112;  fracture  cleavage,  jointing  and 
flow  cleavage  in  graywacke  and 
slate,  65 

Alps,  "decken"  structure  and  folding, 
106,  117,  138;  faults,  60,  137; 
topography  influenced  by  folding 
or  faulting,  140 

Anthracite  -  Crested  Butte  folio, 
faults,  60 

Anorthosite,  Morin,  granulation,  88, 
90 

Appalachians,  northern,  depth  of 
folds,  125 

Appalachians,  southern,  absence  of 
fault  scarps,  59;  compression 
joints,  23;  contrast  between  frac- 
ture and  flow,  13;  determination 
of  fault  displacements,  38;  dis- 
tributive and  thrust  faults,  46,  48, 
137;  folded  thrust  fault  planes,  51, 
54;  folding,  111,  124,  137,  138; 
intimate  relation  of  thrust  faults 
and  overthrust  folds,  51;  sliced 
feldspars  in  micaceous  and  chloritic 
schist,  83 ;  topography  influenced  by 
folding  or  faulting,  140.  See  also 
Piedmont  Plateau 

Arizona,  faults,  44,  57.  See  also 
Clifton,  Globe,  Grand  Canyon 


Atwood,  W.  W.,  60,  133 
Autoclastics,  64 

Baltimore  gneisses  of  Piedmont  Pla- 
teau, 98 

Bannock  overthrust  in  southeastern 
Idaho,  51,  60 

Baraboo  district,  Wisconsin,  folding 
of  slate,  129;  fracture  cleavage  and 
jointing  in  quartzite,  23,  24,  26,  31, 
64,  121;  vertical  section  of  Illinois 
Mine,  129 

Barlow,  A.  E.,  100 

Bascom,  F.,  98 

Basin  Ranges,  faults  in,  43,  44,  56,  57, 
58,  137;  laboratory  study  of  moun- 
tains, 140;  tension  joints  in,  22. 
See  also  Utah,  Nevada,  Arizona 

Basins,  ocean,  141 

Bastin,  E.  S.,  99,  100,  101 

Becker,  G.  F.,  15,  29,  30,  44,  45,  77, 
85 

Bisbee  district,  Arizona,  faults,  53, 
60 

Black  Hills,  South  Dakota,  chloritoid 
crystal  in  micaceous  and  quartzose 
schist,  91;  cleavage,  63, 154;  photo- 
micrograph of  slate,  63 

Blackwelder,  Eliot,  iii,  159,  160 

Box  Canyon,  Colorado,  unconform- 
ity, 156 

Brazil,  determination  of  rocks  by 
washing,  99;  peaks  of  hematite,  151 

Breccias,  64 

Bristol  folio,  Virginia,  folding,  134 

British  Columbia,  folding,  127 

Buckley,  E.  R.,  17 


163 


164 


INDEX 


Buffalo  Mountain,  Tennessee,  theo- 
retical section,  52 

Bullfrog  district,  Nevada,  fault  dis- 
placements, 39;  extension  by  fault- 
ing, 56;  hinge  faults,  42 

Butte  district,  Montana,  determina- 
tion of  fault  displacements,  39 

Cadell,  H.  M.,  49,  51 

California.  See  Mother  Lode,  Santa 
Cruz,  Yosemite  Valley 

California  earthquake,  faults,  44,  50, 
57,  59 

Campbell,  D.  F.,  153 

Carolina  gneiss  of  Piedmont  Plateau, 
98 

Cascade  Mountains,  Washington, 
method  of  determining  depth  af- 
fected by  folds,  125,  127;  cause  of 
uplift,  153 

Chamberlin,  R.  T.,  125,  126,  127,  156 

Chamberlin,  T.  C.,  36,  40,  44,  53, 
54,  121,  125,  127,  139,  142,  145, 
146 

Cleavage  crossing  bedding,  94,  95. 
See  Flow  Cleavage,  Fracture  Cleav- 
age 

Clifton  district,  Arizona,  faults,  42, 
53 

Cloud  Peak-Fort  McKinney  folio, 
folding,  134 

Clough,  C.  T.,  49 

Cobalt  district,  Ontario,  fractures  in 
gabbro,  22;  identification  of  con- 
glomerate, 66;  unconformity,  158 

Coker,  E.  G.,  92 

Colorado.  See  Anthracite-Crested 
Butte,  Box  Canyon,  Georgetown, 
Silverton,  Spanish  Peaks 

Compression  fractures,  16,  21 

Compression  joints,  23 

Conglomerate,  identification  of,  66 

Connecticut.    See  Pomperaug  Valley 

Continents,  141 

Cranberry  folio,  North  Carolina, 
laboratory  study  of  mountains, 
140 


Crosby,  W.  O.,  67 
Crystalloblastic  structure,  77 
Cushing,  H.  P.,  36 
Cuyuna  district,  Minnesota,  observa- 
tions on  folding,  113 

Dale,  T.  Nelson,  26,  27,  31,  64 

Dalles  of  Wisconsin,  drainage  con- 
trolled by  joints,  30;  false  bedding 
in  sandstone,  133 

Daly,  R.  A.,  127 

Dana,  James  D.,  105 

Daubree,  A.,  15,  29 

Day,  Arthur  L.,  29,  77,  85 

Decken  structure,  117 

Deformation,  ultimate  forces  of,  145 

Deformation  and  vulcanism,  154 

Derby,  O.  A.,  99 

Distributive  fault,  48 

Ducktown,  Tennessee,  sedimentary 
origin  of  gneisses,  98 

Dutton,  C.  E.,  146 

Earthquakes,  67;  and  glaciers,  67; 
and  magnetic  disturbances,  70; 
and  rock  density,  70;  and  vulcan- 
ism, 70;  as  cause  and  effect  of  rock 
fractures,  67;  location  of,  73;  pre- 
diction of,  74;  seismograph,  71; 
waves,  72;  waves  in  relation  to 
earth's  interior,  72;  zones,  71 

Emmons,  W.  H.,  39,  42,  56,  98 

Evans,  John  W.,  36 

Faults,  31;  correlation  of,  53;  dis- 
placements, apparent  and  real, 
36,  50;  distributive  thrust,  48; 
evidence  of,  56;  folded,  51;  grading 
into  folds  or  flowage,  51;  hinge,  50; 
laboratory  study,  60;  nomencla- 
ture of,  32;  normal,  39;  normal, 
associated  with  folds,  43;  normal, 
associated  with  igneous  rocks,  42; 
normal,  in  intersecting  systems,  44; 
normal,  in  unfolded  sediments,  43; 
number  of  reverse  and  normal,  54; 
pivotal,  50;  relative  shortening 


INDEX 


165 


and  elongation  of  the  earth's  crust 
by,  55;  reverse  or  thrust,  46;  sur- 
face expression  of,  57.  See  also 
Fractures 

Fissility.    See  Fracture  Cleavage 
Flow    and    fracture,    conditions,    4; 
distribution,   2;  kinds,   2;  surface 
expression  of  zones,  3,  12;  volume 
changes,  11 
Flow  cleavage,   76;  and  folds,   119, 

129;  and  pressure,  83 
Flowage.  See  Rock  Flowage 
Folds,  104;  definitions,  104;  depth, 
124;  differential  movement  on 
limbs,  114;  elements,  104;  field 
observations,  127;  fracture  and 
flow  contrasted,  108;  laboratory 
study,  134;  minor  drag,  114,  128; 
relation  to  cleavage,  119,  128;  re- 
lation to  joints,  121;  relation  to 
faults,  43,  51 ;  strike  and  dip  obser- 
vations, 127 

Forces  of  secondary  deformation,  144 
Fractures,    14,    19;  tension,    14,   20. 

See  also  Joints,  Faults 
Fracture  cleavage,  61;  compression, 
16,  21;  in  relation  to  earthquakes, 
67 
Fracture  and  flow.     See  Flow  and 

Fracture 

Front  Range,  Colorado,  folding,  138 
Front  Range,  Montana,  thrust  faults, 

53 
Futterer,  Karl,  82 

Gadsden  folio,  Alabama,  evidences  of 
fracture  and  flow,  13 

Garrey,  G.  H.,  39,  42,  56,  98 

Geanticlines,  141 

Geikie,  Archibald,  49 

Georgetown  area,  Colorado,  sedi- 
mentary origin  of  gneiss,  98 

Georgia.    See  Rome 

Geosynclines,  141 

Gilbert,  G.  K.,  3,  44,  50,  57,  58,  59, 
65,  75,  146 

Glaciers  and  earthquakes,  67 


Globe  district,  Arizona,  correlation  of 
faults  in,  53 

Gneiss,  criteria  for  origin,  87,  97 

Gneissic  structure,  87 

Gogebic  district,  Michigan,  evidences 
of  fracture  and  flow,  13;  uncon- 
formity, 161 

Goldfield  district,  Nevada,  hinge 
faults  in,  42 

Grand  Canyon,  drainage  controlled 
by  joints,  30,  31 

Granulation  in  rock  flowage,  82 

Grubenmann,  U.,  77 

Gunn,  W.,  49 

Hallock,  William,  7,  9 

Harder,  E.  C.,  31 

Harrisburg,  Pennsylvania,  section 
from,  to  Tyrone,  125,  126 

Hartville  folio,  Wyoming,  uncon- 
formity, 161 

Hayford,  John  F.,  146,  147,  148,  149, 
150 

Heim,  Albert,  3,  106,  117,  118,  140 

Henry  Mountains,  existence  of  zones 
of  fracture  and  flow,  3 

Highlands  of  Scotland,  faults,  39,  49, 
51,  60,  137,  140 

Hinge  fault,  50 

Hinxman,  L.  W.,  49 

Hobbs,  W.  H.,  44 

Holyoke  folio,  Massachusetts,  un- 
conformity, 161 

Hoosac,  Massachusetts,  micaceous 
and  quartzose  schist,  80,  81 

Home,  John,  49 

Hoskins,  L.  M.,  9,  10,  16,  86 

Hotchkiss,  W.  O.,  116 

Hurricane  fault  scarp,  56,  57,  58 

Idaho,  Bannock  overthrust  in  south- 
eastern, 51,  60 

Idiomorphic  texture  in  rock  flowage, 
90 

Illinois  Mine,  Baraboo  district,  Wis- 
consin, vertical  section  of,  129 

Imbricate  structure,  49 


166 


INDEX 


Indian  Territory.    See  Tahlequah 

Iron  Springs  district,  Utah,  hinge 
faults  in,  50;  tension  joints  in, 
23 

Isostasy,  145;  criticism  of,  149; 
Button's  and  Gilbert's  observa- 
tions on,  146;  Hayford's  observa- 
tions on,  146;  in  relation  to  earth 
movements,  148;  in  relation  to 
rigidity  of  rocks,  148 

Isostatic  compensation,  depth  of,  149 

Jaggar,  T.  A.,  36 

Johnston,  John,  76 

Joints,  21,  25;  and  folds,  121;  com- 
pression, 23,  28;  in  intersecting 
systems,  44;  laboratory  work  on, 
31;  of  unknown  origin,  28;  surface 
expression  of,  30;  tension,  22,  26; 
widened  by  growing  crystals,  29. 
See  also  Fractures 

Kaibab  fault,  Utah,  51 
Keith,  Arthur,  51,  52,  96,  98 
Kick,  Friedrich,  4 
Kingston  earthquake,  72 
Knoxville    folio,    Tennessee,    uncon- 
formity, 161 

Laboratory  study  of  faults,  60; 
folds,  134;  joints,  31;  mountains, 
140;  unconformity,  161 

Lake  Superior  Region,  fracture  and 
flow  contrasted,  13;  gneisses  of, 
102;  identification  of  tuffs,  66; 
identification  of  schists,  103 

Laney,  F.  B.,  98 

Laurentian  area  north  of  Montreal, 
leaf  gneiss  from,  89 

Lawson,  A.  C.,  60 

Leaf  gneiss  north  of  Montreal, 
89 

Lehmann,  Johann,  102 

Leith,  C.  K.,  66,  76 

Lewis  and  Livingston  Ranges,  labora- 
tory study  of  mountains,  140; 
thrust  faults,  53 


Lewis,  Harmon,  149,  150 

Limestone  Cove,  Tennessee,  theo- 
retical section,  52 

Lindgren,  Waldemar,  42 

Little  Belt  Mountains  folio,  Mon- 
tana, laboratory  study  of  moun- 
tains, 140 

Little  Falls,  Minnesota,  slaty  cleav- 
age crossing  bedding,  94 

Livingston  folio,  Montana,  labora- 
tory study  of  mountains,  140;  un- 
conformity, 161 

Livingston  Range.    See  Lewis 

Lugeon,  M.,  117 

Magnetic  disturbances  and  earth- 
quakes, 70 

Maine,  granites,  31 

Mansfield,  G.  R.,  51,  60 

Marquette  district,  Michigan,  cor- 
relation of  structures,  113,  115; 
evidences  of  fracture  and  flow,  13; 
unconformity,  161 

Martin,  Lawrence,  58,  70 

Massachusetts,  granites,  31.  See 
also  Holyoke,  Hoosac 

Mathews,  E.  B.,  98 

Maynardville,  folio,  Tennessee,  fold- 
ing, 134 

Mead,  W.  J.,  iii,  119 

Mediterranean  earthquake  zone,  71 

Menominee  district,  Michigan,  fold- 
ing, 117,  134;  origin  of  green 
schists,  101 

Mercersburg-Chambersburg  folio, 
Pennsylvania,  unconformity,  161 

Mesabi  district,  unconformity,  161 

Messina  earthquake,  71 

Michigan.  See  Gogebic,  Lake  Supe- 
rior, Marquette,  Menominee 

Milch,  L.,  77 

Milne,  John,  69,  72,  73 

Milwaukee  folio,  Wisconsin,  uncon- 
formity, 161 

Minnesota.  See  Cuyuna,  Lake  Su- 
perior, Little  Falls,  Mesabi,  St. 
Louis,  Vermilion 


INDEX 


167 


Mississippi  Valley,  basal  conglomer- 
ate, 158 

Montana.  See  Front  Range,  Lewis 
and  Livingston  Ranges,  Little 
Belt  Mountains,  Three  Forks 

Monterey  folio,  Virginia,  folding,  134; 
laboratory  study  of  mountains,  140 

Montessus  de  Ballore,  F.  de,  71 

Montreal,  leaf  gneiss,  89 

Morin  anorthosite,  granulation  of,  88 

Morristown  folio,  Tennessee,  faults, 
60;  folds,  134 

Mother  Lode  district,  California, 
widening  of  joints,  29 

Mount  Mitchell  folio,  North  Caro- 
lina, folding,  134 

Mount  Stuart  folio,  Washington,  un- 
conformity, 161 

Mountains,  136;  laboratory  study, 
140;  localization  of,  138;  types  of, 
136 

Negative  elements,  141 

Nevada,  faults  in,  44,  57.  See  also 
Bullfrog,  Goldfield,  Great  Basin, 
Tonopah 

New  Hampshire,  granites,  31 

New  Jersey.    See  Trenton 

New  Mexico.    See  Watrous 

Nicolson,  J.  T.,  92 

Normal  faults,  39.  See  also  Faults, 
normal 

North  Carolina.  See  Cranberry, 
Mt.  Mitchell,  Pisgah,  Roan  Moun- 
tain 

Ocean  basins,  141 

Oelrichs  folio,  South  Dakota,  folding, 

134 

Ontario.    See  Cobalt 
Oregon.    See  Cascade 
Ozarks,    topography    influenced    by 

folding  or  faulting,  140.    See  also 

Tahlequah 

Pacific  earthquake  zone,  71 
Peach,  B.  N.,  49 


Pennsylvania,  method  of  determin- 
ing depth  affected  by  folds,  125. 
See  also  Mercersburg,  Piedmont 
Plateau,  Tyrone 

Pfaff,  F.,  7,  9 

Piedmont  Plateau,  cleavage,  120; 
folding,  111;  fracture  and  flow  con- 
trasted, 13;  sedimentary  origin  of 
gneisses,  98.  See  also  Appalachi- 
ans, southern 

Pisgah  folio,  North  Carolina,  evi- 
dences of  fracture  and  flow,  13 

Pivotal  fault,  50 

Plateaus,  141 

Pomperaug  Valley,  normal  faults  in 
intersecting  systems,  44 

Porphyritic  texture  in  rock  flowage, 
90 

Positive  elements,  141 

Protoclastic  structure,  87 

Putnam,  G.  R.,  146 

Ransome,  F.  L.,  39,  41,  42,  56 

Reverse  faults.    See  Faults 

Rhode  Island,  granites,  31 

Richards,  R.  W.,  51,  60 

Ringgold,  folio,  Georgia,  folding,  134 

Ripple  marks,  131 

Roan  Mountain  folio,  North  Caro- 
lina, evidences  of  fracture  and 
flow,  13;  faults,  48,  52,  60;  folded 
thrust  fault  planes,  51 

Rock  density  and  earthquakes,  70 

Rock  flowage,  76;  evidence  for  the 
existence  of  zone,  3;  grading  into 
faulting,  51;  obliteration  of  tex- 
tures, 93;  without  cleavage,  92 

Rome  folio,  Georgia,  laboratory 
study  of  mountains,  140;  uncon- 
formity, 161 

Rotation  in  rock  flowage,  82 

Salisbury,  R.  D.,  54,  60, 133, 139, 142, 
145,  146 

Santa  Cruz  folio,  California,  relations 
of  valleys  and  lakes  to  fault  dis- 
placements, 60 


168 


INDEX 


Saxony  area,  development  of  gneisses 
from  granites,  102 

Scandinavian  Highlands,  faulting, 
137 

Schardt,  Hans,  117 

Schists,  identification,  97 

Schuppen  structure,  48,  49 

Scotland.    See  Highlands 

Sierra  Nevada  Mountains,  faults,  44, 
137 

Silverton  folio,  Colorado,  faults, 
60 

Simplon  Tunnel,  rock  flowage,  1 

Slicing  in  rock  flowage,  83 

South  Dakota.  See  Black  Hills, 
Oelrichs 

Southern  Appalachians.  See  Appala- 
chians 

Spanish  Peaks  folio,  Colorado,  labo- 
ratory study  of  mountains,  140 

Spurr,  J.  E.,  36,  42,  43,  58,  98,  140, 
157 

Steidtmann,  E.,  26,  31 

Steinmann,  G.,  140 

St.  Louis  slates,  cleavage  crossing 
bedding,  95 

Suess,  Edward,  139 

Sundance  folio,  Wyoming,  laboratory 
study  of  mountains,  140;  folding 
shown,  134 

Tahlequah  folio,  Indian  Territory, 
laboratory  study  of  mountains, 
140 

Tarr,  R.  S.,  70 

Teall,  J.  J.  H.,  49 

Tennessee.  See  Bristol,  Buffalo 
Mountain,  Knoxville,  Limestone 
Cove,  Maynardville,  Morristown, 
Ringgold,  Walland 

Tension,  causes  of,  152;  fractures,  14, 
20;  joints,  22.  See  also  Joints 

Terlingua,  Texas,  topographic  map, 
faulting,  59 

Three  Forks  folio,  Montana,  folding, 
134;  laboratory  study  of  moun- 
tains, 140;  unconformity,  161 


Thrust  faults.    See  Faults 

Tolman,  C.  F.,  36 

Tonopah  district,  Nevada,  faults,  22, 
42,  43,  53 

Trenton  folio,  New  Jersey,  uncon- 
formity, 161 

Trueman,  J.  D.,  78,  89,  100 

Tyrone,  Pennsylvania,  section,  125, 
126 

Unconformity,  156;  identification, 
157;  interpretation,  159;  labora- 
tory study,  161 

Uplifts,  actual  and  apparent,  143 
Utah,  faults,  44,  57.    See  also  Hurri- 
cane, Iron  Springs,  Kaibab 

Van  Hise,  C.  R.,  iii,  3,  9,  10,  22, 
28,  53,  61,  66,  105,  107,  110,  121, 
131 

Vermilion  district,  Minnesota,  fold- 
ing, 117,  128;  pseudo-conglomer- 
ates, 66 

Vermont,  granites  of,  26,  31 

Virginia.    See  Monterey 

Vulcanism  and  deformation,  154;  and 
earthquakes,  70 

Walland,    Tennessee,    cleavage    and 

bedding,  96 
Wasatch  Mountains,  Hurricane  fault 

scarp,  56,  57,  58;  normal  faults,  44, 

137 
Washington,  D.  C.,  identification  of 

gneiss,  98;  unconformity,  161 
Washington     State.      See     Cascade 

Range,  Mt.  Stuart 
Watrous,  New  Mexico,  joints,  31 
Weed,  W.  H.,  39 
Weidman,  Samuel,  129 
Wenatchee-Chelan  district.    See  Cas- 
cade Range 

Williams,  G.  H.,  99,  101 
Willis,  Bailey,  33,  36,  46,  47,  53,  60, 

104,  111,  112,  122,  123,  124,  125, 

127,  135,  141 


INDEX 


169 


Wisconsin,    southwestern   joint   sys-      Yakutat  Bay,  Alaska,  earthquakes  at, 
tern,  31.    See  also  Baraboo,  Dalles,  70 


Milwaukee 
Wright,  F.  E.,  84 
Wyoming.     See  Cloud  Peak,  Hart- 

ville,  Sundance 


Yosemite  Valley,  drainage  controlled 
by  joints,  30,  31 

Zone  of  flow  contrasted  with  zone  of 
fracture,  108 


14  DAY  USE 

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